ARM: shmobile: r8a7790: Rename mmcif node to mmc

Node names should describe the function of the device, not its IP core
name.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Tested-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>