[ARM] pxa: enable check_scoop_reg() only if CONFIG_PM is set.

This function gets only called from scoop_suspend() and scoop_resume() which are
only built in if we have CONFIG_PM set. Make it the same for check_scoop_reg().

This fixes the following warning:
arch/arm/common/scoop.c:143: warning: ‘check_scoop_reg’ defined but not used

Signed-off-by: Stefan Schmidt <stefan@datenfreihafen.org>
Signed-off-by: Eric Miao <eric.y.miao@gmail.com>