ACPI / sleep: pm_power_off needs more sanity checks to be installed

Sleep control and status registers need santity checks as well before
ACPI installs acpi_power_off to pm_power_off hook. The checking code in
acpi_enter_sleep_state() is too late, we should not allow a not-working
pm_power_off function to be hooked up.

Signed-off-by: Aubrey Li <aubrey.li@intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>