gpio: Convert tps65910 to a platform driver

Make the gpio-tps65910 as platform driver and register
this from tps65910 core driver as mfd sub device.

Signed-off-by: Laxman Dewangan <ldewangan@nvidia.com>
Acked-by: Grant Likely <grant.likely@secretlab.ca>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>