ARM: shmobile: r8a7791: Support Core-Standby for Suspend to RAM

Add r8a7791 Core-Standby state for Suspend to RAM support.

Signed-off-by: Keita Kobayashi <keita.kobayashi.ym@renesas.com>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>