ARM: shmobile: Introduce ARCH_SHMOBILE_MULTI

Add ARCH_SHMOBILE_MULTI to mach-shmobile that can be used
to enable ARCH_MULTIPLATFORM on selected SoCs and boards.

The headers stay under arch/arm/mach-shmobile/include/mach/
for now, they can and will be migrated independently over time.

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