[ACPI] re-enable platform-specific hotkey drivers by default

When both platform-specific and generic drivers exist,
enable generic over-ride with "acpi_generic_hotkey".

http://bugzilla.kernel.org/show_bug.cgi?id=4953

Signed-off-by: Luming Yu <luming.yu@intel.com>
Signed-off-by: Len Brown <len.brown@intel.com>