ARM: SH-Mobile: sh73a0: Add CPU Hotplug

Add the capability to add and remove CPUs on the fly.
The Cortex-A9 offers the possibility to take single cores out of the
MP Core. We add this capabilty taking care that caches are kept
coherent. For verifying the shutdown we rely on the internal SH73A0
Power Status Register PSTR.

Signed-off-by: Bastian Hecht <hechtb+renesas@gmail.com>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>