Davinci SPI controller device bindings

Links on DM:
Keystone 2 - https://www.ti.com/lit/ug/sprugp2a/sprugp2a.pdf
dm644x - https://www.ti.com/lit/ug/sprue32a/sprue32a.pdf
OMAP-L138/da830 - http://www.ti.com/lit/ug/spruh77a/spruh77a.pdf

Required properties:
- #address-cells: number of cells required to define a chip select
	address on the SPI bus. Should be set to 1.
- #size-cells: should be zero.
- compatible:
	- "ti,dm6441-spi" for SPI used similar to that on DM644x SoC family
	- "ti,da830-spi" for SPI used similar to that on DA8xx SoC family
	- "ti,keystone-spi" for SPI used similar to that on Keystone2 SoC
		family
- reg: Offset and length of SPI controller register space
- num-cs: Number of chip selects. This includes internal as well as
	GPIO chip selects.
- ti,davinci-spi-intr-line: interrupt line used to connect the SPI
	IP to the interrupt controller within the SoC. Possible values
	are 0 and 1. Manual says one of the two possible interrupt
	lines can be tied to the interrupt controller. Set this
	based on a specific SoC configuration.
- interrupts: interrupt number mapped to CPU.
- clocks: spi clk phandle
          For 66AK2G this property should be set per binding,
          Documentation/devicetree/bindings/clock/ti,sci-clk.yaml

SoC-specific Required Properties:

The following are mandatory properties for Keystone 2 66AK2G SoCs only:

- power-domains:	Should contain a phandle to a PM domain provider node
			and an args specifier containing the SPI device id
			value. This property is as per the binding,

Optional:
- cs-gpios: gpio chip selects
	For example to have 3 internal CS and 2 GPIO CS, user could define
	cs-gpios = <0>, <0>, <0>, <&gpio1 30 0>, <&gpio1 31 0>;
	where first three are internal CS and last two are GPIO CS.

Optional properties for slave devices:
SPI slave nodes can contain the following properties.
Not all SPI Peripherals from Texas Instruments support this.
Please check SPI peripheral documentation for a device before using these.

- ti,spi-wdelay : delay between transmission of words
	(SPIFMTn.WDELAY, SPIDAT1.WDEL) must be specified in number of SPI module
	clock periods.

	delay = WDELAY * SPI_module_clock_period + 2 * SPI_module_clock_period

Below is timing diagram which shows functional meaning of
"ti,spi-wdelay" parameter.

             +-+ +-+ +-+ +-+ +-+                           +-+ +-+ +-+
SPI_CLK      | | | | | | | | | |                           | | | | | |
  +----------+ +-+ +-+ +-+ +-+ +---------------------------+ +-+ +-+ +-

SPI_SOMI/SIMO+-----------------+                           +-----------
  +----------+ word1           +---------------------------+word2
             +-----------------+                           +-----------
                                          WDELAY
                                <-------------------------->

Example of a NOR flash slave device (n25q032) connected to DaVinci
SPI controller device over the SPI bus.

spi0:spi@20bf0000 {
	#address-cells			= <1>;
	#size-cells			= <0>;
	compatible			= "ti,dm6446-spi";
	reg				= <0x20BF0000 0x1000>;
	num-cs				= <4>;
	ti,davinci-spi-intr-line	= <0>;
	interrupts			= <338>;
	clocks				= <&clkspi>;

	flash: n25q032@0 {
		#address-cells = <1>;
		#size-cells = <1>;
		compatible = "st,m25p32";
		spi-max-frequency = <25000000>;
		reg = <0>;
		ti,spi-wdelay = <8>;

		partition@0 {
			label = "u-boot-spl";
			reg = <0x0 0x80000>;
			read-only;
		};

		partition@1 {
			label = "test";
			reg = <0x80000 0x380000>;
		};
	};
};