ARM: mach-shmobile: sh73a0 SMP support

Add SMP support for ag5evm and the sh73a0 processor.

Onlining and offlining works well, but at this point
offlined processor cores are not put into sleep mode.

There is no spinlock for syncing the secondary core
with the first one in this implementation. The code
instead relies on the cpu_online() check in __cpu_up().

Signed-off-by: Magnus Damm <damm@opensource.se>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>