ARM: shmobile: r8a7794: add MMCIF DT support

Define the generic R8A7794 part of the MMCIF0 device node.

Based on the orginal patch by Shinobu Uehara <shinobu.uehara.xc@renesas.com>.

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