mmc: sdhci-cadence: add Cadence SD4HC support

Add a driver for the Cadence SD4HC SD/SDIO/eMMC Controller.

For SD, it basically relies on the SDHCI standard code.
For eMMC, this driver provides some callbacks to support the
hardware part that is specific to this IP design.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Acked-by: Adrian Hunter <adrian.hunter@intel.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>

Documentation/devicetree/bindings/mmc/sdhci-cadence.txt

@@ -0,0 +1,30 @@
+* Cadence SD/SDIO/eMMC Host Controller
+
+Required properties:
+- compatible: should be "cdns,sd4hc".
+- reg: offset and length of the register set for the device.
+- interrupts: a single interrupt specifier.
+- clocks: phandle to the input clock.
+
+Optional properties:
+For eMMC configuration, supported speed modes are not indicated by the SDHCI
+Capabilities Register.  Instead, the following properties should be specified
+if supported.  See mmc.txt for details.
+- mmc-ddr-1_8v
+- mmc-ddr-1_2v
+- mmc-hs200-1_8v
+- mmc-hs200-1_2v
+- mmc-hs400-1_8v
+- mmc-hs400-1_2v
+
+Example:
+	emmc: sdhci@5a000000 {
+		compatible = "cdns,sd4hc";
+		reg = <0x5a000000 0x400>;
+		interrupts = <0 78 4>;
+		clocks = <&clk 4>;
+		bus-width = <8>;
+		mmc-ddr-1_8v;
+		mmc-hs200-1_8v;
+		mmc-hs400-1_8v;
+	};