ARM: shmobile: r8a7778: add support MMC driver

Add a platform device for the r8a7778 MMC.

Signed-off-by: Yusuke Goda <yusuke.goda.sx@renesas.com>
Signed-off-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>