ARM: shmobile: Rework sh7372 sleep code to use virt_to_phys()

Instead of having a hard coded virt-to-phys address
conversion code in sleep-sh7372.S, rework the code
to do the conversion in C using virt_to_phys().

This removes the need for PLAT_PHYS_OFFSET which
in turn is needed for ARCH_MULTIPLATFORM.

Signed-off-by: Magnus Damm <damm@opensource.se>
[horms+renesas@verge.net.au: squashed in build fix posted as
 "ARM: shmobile: sh7372 build fix for SUSPEND=n && CPU_IDLE=n"]
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>