sh: let ARCH_SHMOBILE select PM and PM_RUNTIME

With the Runtime PM driver changes in place, we must have
Runtime PM support in place. Otherwise there is no way to
enable clocks to the Runtime PM enabled hardware blocks.
This patch makes Runtime PM mandatory on SuperH Mobile.

Signed-off-by: Magnus Damm <damm@igel.co.jp>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>