arm64: dts: renesas: r8a77970: add MMC support

Define the generic R8A77970 part of the MMC0 (SDHI2) device node.

Based on the original (and large) patches by Vladimir Barinov.

Signed-off-by: Vladimir Barinov <vladimir.barinov@cogentembedded.com>
Signed-off-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>