ARM: shmobile: Move headsmp-sh73a0.S to headsmp-scu.S

Rename headsmp-sh73a0.S into headsmp-scu.S and
introduce shmobile_secondary_vector_scu().

The goal is to be able to share the function
above between all mach-shmobile SoCs that use
SCU for SMP. So far only sh73a0 use this.

At this time the SCU base address is still hard
coded in headsmp-scu.S to 0xf0000000, but this
will be changed in the future.

Signed-off-by: Magnus Damm <damm@opensource.se>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>