dt-binding: mmc: sunxi: add H6 compatible (with A64 fallback)

The MMC controllers on H6 is similar to the ones on A64, but with some
new features.

Add compatible strings for them (with the A64 compatible strings as
fallback, in order to make them to work with A64 drivers).

Signed-off-by: Icenowy Zheng <icenowy@aosc.io>
Signed-off-by: Maxime Ripard <maxime.ripard@bootlin.com>