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>