PM: Make sysrq-o be available for CONFIG_PM unset

After commit 1eb208ae, "PM: Make
CONFIG_PM depend on (CONFIG_PM_SLEEP || CONFIG_PM_RUNTIME)", the
files under kernel/power are not built unless CONFIG_PM_SLEEP or
CONFIG_PM_RUNTIME is set.  In particular, this causes
kernel/power/poweroff.c to be omitted, even though it should be
compiled, because CONFIG_MAGIC_SYSRQ is set.

Fix the problem by causing kernel/power/Makefile to be processed
for CONFIG_PM unset too.

Reported-and-tested-by: Phil Oester <kernel@linuxace.com>
Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>