PM / shmobile: Fix build of sh7372_pm_init() for CONFIG_PM unset

Fix build regression introduced by commit 056879d2


(ARM: mach-shmobile: sh7372 A3SP no_suspend_console fix) by moving
the intialization of the A3SP domain to a separate function and
providing an empty definition of it for CONFIG_PM unset.

Reported-and-tested-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>