ARM: shmobile: r8a7790 CA15-SCU enablement

Power on CA15 SCU in case of booting from a CA7 core.

Signed-off-by: Keita Kobayashi <keita.kobayashi.ym@renesas.com>
[damm@opensource.se: Converted to use broken out SYSC code]
Signed-off-by: Magnus Damm <damm@opensource.se>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>