ARM: Make consolidated PM sleep code depend on PM_SLEEP

CONFIG_PM is now set whenever we support either runtime PM in addition
to suspend and hibernate.  This causes build errors when runtime PM is
enabled on a platform, but the CPU does not have the appropriate support
for suspend.

So, switch this code to use CONFIG_PM_SLEEP rather than CONFIG_PM to
allow runtime PM to be enabled without causing build errors.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>