ARM: shmobile: r8a7794: add SDHI DT support

Define the generic R8A7794 parts of the SDHI[012] device nodes.

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

Signed-off-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Acked-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>