These properties are common to multiple MMC host controllers. Any host
that requires the respective functionality should implement them using
these definitions.

Interpreted by the OF core:
- reg: Registers location and length.
- interrupts: Interrupts used by the MMC controller.

Required properties:
- bus-width: Number of data lines, can be <1>, <4>, or <8>

Optional properties:
- cd-gpios: Specify GPIOs for card detection, see gpio binding
- wp-gpios: Specify GPIOs for write protection, see gpio binding
- cd-inverted: when present, polarity on the cd gpio line is inverted
- wp-inverted: when present, polarity on the wp gpio line is inverted
- non-removable: non-removable slot (like eMMC)
- max-frequency: maximum operating clock frequency

Example:

sdhci@ab000000 {
	compatible = "sdhci";
	reg = <0xab000000 0x200>;
	interrupts = <23>;
	bus-width = <4>;
	cd-gpios = <&gpio 69 0>;
	cd-inverted;
	wp-gpios = <&gpio 70 0>;
	max-frequency = <50000000>;
}