mmc: dw_mmc-pltfm: add Rockchip variant

Cortex-A9 SoCs from Rockchip use a slightly modified variant of dw_mmc
controllers that seems to require the SDMMC_CMD_USE_HOLD_REG bit to
always be set.

There also seem to be no other modifications (additional register etc)
present, so to keep the footprint low, add this small variant to the
pltfm driver.

Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Acked-by: Seungwon Jeon <tgih.jun@samsung.com>
Signed-off-by: Chris Ball <cjb@laptop.org>

Documentation/devicetree/bindings/mmc/rockchip-dw-mshc.txt

@@ -0,0 +1,23 @@
+* Rockchip specific extensions to the Synopsis Designware Mobile
+  Storage Host Controller
+
+The Synopsis designware mobile storage host controller is used to interface
+a SoC with storage medium such as eMMC or SD/MMC cards. This file documents
+differences between the core Synopsis dw mshc controller properties described
+by synopsis-dw-mshc.txt and the properties used by the Rockchip specific
+extensions to the Synopsis Designware Mobile Storage Host Controller.
+
+Required Properties:
+
+* compatible: should be
+	- "rockchip,rk2928-dw-mshc": for Rockchip RK2928 and following
+
+Example:
+
+	rkdwmmc0@12200000 {
+		compatible = "rockchip,rk2928-dw-mshc";
+		reg = <0x12200000 0x1000>;
+		interrupts = <0 75 0>;
+		#address-cells = <1>;
+		#size-cells = <0>;
+	};