ARM: shmobile: Make rmobile_init_pm_domain() static

Since rmobile_init_pm_domain() is not called anywhere outside of
arch/arm/mach-shmobile/pm-rmobile.c any more, it can be made static
and its header may be removed from pm-rmobile.h.  Modify the code
accordingly.

Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>
Acked-by: Magnus Damm <damm@opensource.se>