Documentation: dt: Add new compatible to sunxi mmc driver bindings

Signed-off-by: NIcenowy Zheng <icenowy@aosc.xyz>
Signed-off-by: NUlf Hansson <ulf.hansson@linaro.org>

Documentation/devicetree/bindings/mmc/sunxi-mmc.txt

@@ -13,6 +13,7 @@ Required properties:
    * "allwinner,sun5i-a13-mmc"
    * "allwinner,sun7i-a20-mmc"
    * "allwinner,sun9i-a80-mmc"
+   * "allwinner,sun50i-a64-mmc"
  - reg : mmc controller base registers
  - clocks : a list with 4 phandle + clock specifier pairs
  - clock-names : must contain "ahb", "mmc", "output" and "sample"