ST DWC3 glue logic

This file documents the parameters for the dwc3-st driver.
This driver controls the glue logic used to configure the dwc3 core on
STiH407 based platforms.

Required properties:
 - compatible	: must be "st,stih407-dwc3"
 - reg		: glue logic base address and USB syscfg ctrl register offset
 - reg-names	: should be "reg-glue" and "syscfg-reg"
 - st,syscon	: should be phandle to system configuration node which
		  encompasses the glue registers
 - resets	: list of phandle and reset specifier pairs. There should be two entries, one
		  for the powerdown and softreset lines of the usb3 IP
 - reset-names	: list of reset signal names. Names should be "powerdown" and "softreset"
See: Documentation/devicetree/bindings/reset/st,stih407-powerdown.yaml
See: Documentation/devicetree/bindings/reset/reset.txt

 - #address-cells, #size-cells : should be '1' if the device has sub-nodes
   with 'reg' property

 - pinctl-names	: A pinctrl state named "default" must be defined
See: Documentation/devicetree/bindings/pinctrl/pinctrl-bindings.txt

 - pinctrl-0	: Pin control group
See: Documentation/devicetree/bindings/pinctrl/pinctrl-bindings.txt

 - ranges	: allows valid 1:1 translation between child's address space and
		  parent's address space

Sub-nodes:
The dwc3 core should be added as subnode to ST DWC3 glue as shown in the
example below. The DT binding details of dwc3 can be found in:
Documentation/devicetree/bindings/usb/snps,dwc3.yaml

NB: The dr_mode property described in [1] is NOT optional for this driver, as the default value
is "otg", which isn't supported by this SoC. Valid dr_mode values for dwc3-st are either "host"
or "device".

[1] Documentation/devicetree/bindings/usb/usb-drd.yaml

Example:

st_dwc3: dwc3@8f94000 {
	compatible	= "st,stih407-dwc3";
	reg		= <0x08f94000 0x1000>, <0x110 0x4>;
	reg-names	= "reg-glue", "syscfg-reg";
	st,syscfg	= <&syscfg_core>;
	resets		= <&powerdown STIH407_USB3_POWERDOWN>,
			  <&softreset STIH407_MIPHY2_SOFTRESET>;
	reset-names	= "powerdown", "softreset";
	#address-cells	= <1>;
	#size-cells	= <1>;
	pinctrl-names	= "default";
	pinctrl-0	= <&pinctrl_usb3>;
	ranges;

	dwc3: dwc3@9900000 {
		compatible	= "snps,dwc3";
		reg		= <0x09900000 0x100000>;
		interrupts	= <GIC_SPI 155 IRQ_TYPE_NONE>;
		dr_mode		= "host";
		phy-names	= "usb2-phy", "usb3-phy";
		phys		= <&usb2_picophy2>, <&phy_port2 PHY_TYPE_USB3>;
	};
};