TPS65910: GPIO: Add GPIO driver

TPS65910 has one configurable GPIO that can be used for several
purposes. Subsequent versions of the TPS chip support more than
one GPIO.

Signed-off-by: Graeme Gregory <gg@slimlogic.co.uk>
Signed-off-by: Jorge Eduardo Candelaria <jedu@slimlogic.co.uk>
Acked-by: Grant Likely <grant.likely@secretlab.ca>
Signed-off-by: Liam Girdwood <lrg@slimlogic.co.uk>