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>

Card detection:
If no property below is supplied, standard SDHCI card detect is used.
Only one of the properties in this section should be supplied:
  - broken-cd: There is no card detection available; polling must be used.
  - cd-gpios: Specify GPIOs for card detection, see gpio binding
  - non-removable: non-removable slot (like eMMC); assume always present.

Optional properties:
- 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
- max-frequency: maximum operating clock frequency
- no-1-8-v: when present, denotes that 1.8v card voltage is not supported on
  this system, even if the controller claims it is.

Optional SDIO properties:
- keep-power-in-suspend: Preserves card power during a suspend/resume cycle
- enable-sdio-wakeup: Enables wake up of host system on SDIO IRQ assertion

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>;
	keep-power-in-suspend;
	enable-sdio-wakeup;
}