gpiolib: Ensure struct gpio is always defined

Currently struct gpio is only defined when using gpiolib which makes the
stub gpio_request_array() much less useful in drivers than is ideal as
they can't work with struct gpio.  Since there are no other definitions
in kernel instead make the define always available no matter if gpiolib
is selectable or selected, ensuring that drivers can always use the
type.

Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>