gpio: enable callback to specify pin in addition to pin_mask

There are GPIO controllers that can control more than 32 pins.
The pin_mask is no longer adequate in this case. This wraps
the pin_mask in a union together with a field named 'pin'.
The driver is responsible for choosing whether to use
pin_mask or pin.

Signed-off-by: Daniel Leung <daniel.leung@intel.com>