ARM: shmobile: r8a7791 SYSC setup code

Add r8a7791 SYSC power management support.

Signed-off-by: Keita Kobayashi <keita.kobayashi.ym@renesas.com>
Acked-by: Magnus Damm <damm+renesas@opensource.se>
[horms+renesas@verge.net.au: rebased]
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>