sunxi: add MMC support for H6

The Allwinner H6 SoC has 3 MMC controllers like the ones in A64, with
the MMC2 come with the capability to do crypto by EMCE.

Add MMC support for H6. EMCE support is not added yet.

Signed-off-by: Icenowy Zheng <icenowy@aosc.io>
Reviewed-by: Jagan Teki <jagan@openedev.com>
Tested-by: Jagan Teki <jagan@amarulasolutions.com>