dt-bindings: mmc: sunxi: Document A64 eMMC compatible

We introduced recently a new compatible to deal with the A64 eMMC
controller, let's document its binding.

Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Acked-by: Chen-Yu Tsai <wens@csie.org>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Ulf 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-emmc"
    * "allwinner,sun50i-a64-mmc"
  - reg : mmc controller base registers
  - clocks : a list with 4 phandle + clock specifier pairs