ARM: shmobile: Initial r8a7794 SoC support

Initial support for the r8a7794 SoC, based on work by Hisashi Nakamura.

Signed-off-by: Ulrich Hecht <ulrich.hecht+renesas@gmail.com>
Acked-by: Magnus Damm <damm+renesas@opensource.se>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>