ARM: shmobile: r8a7740: Add A3SM pm domain support

Add support for the A3SM power domain, and hook it up as a subdomain of
A4S.

This domain contains the System CPU (Cortex-A9) hardware block. Hence
move the special CPU handling from A4S to A3SM.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>