ARM: shmobile: Only select PM_RMOBILE for legacy case

Adjust the ARM mach-shmobile Kconfig bits to only
select PM_RMOBILE in case of legacy. This allows us
to enable partial multiplatform support for various
SoCs before Power Domains DT bindings are finalized.

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