tps65912: gpio: add gpio driver

TPS65912 has five GPIOs that can be configured for different
purposes.

Signed-off-by: Margarita Olaya Cabrera <magi@slimlogic.co.uk>
Acked-by: Grant Likely <grant.likely@secretlab.ca>
Acked-by: Liam Girdwood <lrg@ti.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>