UniPhier SD/eMMC controller

Required properties:
- compatible: should be one of the following:
    "socionext,uniphier-sd-v2.91"  - IP version 2.91
    "socionext,uniphier-sd-v3.1"   - IP version 3.1
    "socionext,uniphier-sd-v3.1.1" - IP version 3.1.1
- reg: offset and length of the register set for the device.
- interrupts: a single interrupt specifier.
- clocks: a single clock specifier of the controller clock.
- reset-names: should contain the following:
    "host"   - mandatory for all versions
    "bridge" - should exist only for "socionext,uniphier-sd-v2.91"
    "hw"     - should exist if eMMC hw reset line is available
- resets: a list of reset specifiers, corresponding to the reset-names

Optional properties:
- pinctrl-names: if present, should contain the following:
    "default" - should exist for all instances
    "uhs"     - should exist for SD instance with UHS support
- pinctrl-0: pin control state for the default mode
- pinctrl-1: pin control state for the UHS mode
- dma-names: should be "rx-tx" if present.
  This property can exist only for "socionext,uniphier-sd-v2.91".
- dmas: a single DMA channel specifier
  This property can exist only for "socionext,uniphier-sd-v2.91".
- bus-width: see mmc.txt
- cap-sd-highspeed: see mmc.txt
- cap-mmc-highspeed: see mmc.txt
- sd-uhs-sdr12: see mmc.txt
- sd-uhs-sdr25: see mmc.txt
- sd-uhs-sdr50: see mmc.txt
- cap-mmc-hw-reset: should exist if reset-names contains "hw". see mmc.txt
- non-removable: see mmc.txt

Example:

	sd: sdhc@5a400000 {
		compatible = "socionext,uniphier-sd-v2.91";
		reg = <0x5a400000 0x200>;
		interrupts = <0 76 4>;
		pinctrl-names = "default", "uhs";
		pinctrl-0 = <&pinctrl_sd>;
		pinctrl-1 = <&pinctrl_sd_uhs>;
		clocks = <&mio_clk 0>;
		reset-names = "host", "bridge";
		resets = <&mio_rst 0>, <&mio_rst 3>;
		dma-names = "rx-tx";
		dmas = <&dmac 4>;
		bus-width = <4>;
		cap-sd-highspeed;
		sd-uhs-sdr12;
		sd-uhs-sdr25;
		sd-uhs-sdr50;
	};
