ARM: shmobile: r8a7794: add GPIO DT support

Describe GPIO[0-6] controllers in the R8A7794 device tree.

Based on original patch by Hisashi Nakamura <hisashi.nakamura.ak@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>