ARM: shmobile: Expose shmobile_invalidate_start()

Expose shmobile_invalidate_start() in common.h for
mach-shmobile. This function will be used for boot of
secondary processors on future non-SCU SMP platforms.

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