langwell_gpio: add runtime pm support

While this is essentially a no-op for this driver, it has the
side effect of letting the PMU driver snoop D3 requests from
the PCI core for this driver.

This is only for langwell, not for whitney point.

Signed-off-by: Kristen Carlson Accardi <kristen@linux.intel.com>
Signed-off-by: Alan Cox <alan@linux.intel.com>
Signed-off-by: Dirk Brandewie <dirk.brandewie@gmail.com>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>