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.

Card detection:
If no property below is supplied, host native 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:
- bus-width: Number of data lines, can be <1>, <4>, or <8>.  The default
  will be <1> if the property is absent.
- wp-gpios: Specify GPIOs for write protection, see gpio binding
- cd-inverted: when present, polarity on the CD line is inverted. See the note
  below for the case, when a GPIO is used for the CD line
- wp-inverted: when present, polarity on the WP line is inverted. See the note
  below for the case, when a GPIO is used for the WP line
- 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.

*NOTE* on CD and WP polarity. To use common for all SD/MMC host controllers line
polarity properties, we have to fix the meaning of the "normal" and "inverted"
line levels. We choose to follow the SDHCI standard, which specifies both those
lines as "active low." Therefore, using the "cd-inverted" property means, that
the CD line is active high, i.e. it is high, when a card is inserted. Similar
logic applies to the "wp-inverted" property.

CD and WP lines can be implemented on the hardware in one of two ways: as GPIOs,
specified in cd-gpios and wp-gpios properties, or as dedicated pins. Polarity of
dedicated pins can be specified, using *-inverted properties. GPIO polarity can
also be specified using the OF_GPIO_ACTIVE_LOW flag. This creates an ambiguity
in the latter case. We choose to use the XOR logic for GPIO CD and WP lines.
This means, the two properties are "superimposed," for example leaving the
OF_GPIO_ACTIVE_LOW flag clear and specifying the respective *-inverted
property results in a double-inversion and actually means the "normal" line
polarity is in effect.

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;
}