ACPI: make ACPI button funcs no-ops if not built in

Yakui pointed out that we don't properly no-op the ACPI button routines
if the button driver isn't built in.  This will cause problems if ACPI
is disabled, so provide stub functions in that case.

Reported-by: ykzhao <yakui.zhao@intel.com>
Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>