mfd: Register tps65910 gpios as an mfd device

As gpio support for tps65910 is on gpio driver, registering
gpio support as the mfd sub devices instead of calling gpio_init()
from the core probe.

Signed-off-by: Laxman Dewangan <ldewangan@nvidia.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>