mfd: Cache tps65910 register when we need it

During regmap initialization, we do not provide the default value and
hence in place of caching register during regmap_init(), cache it
when actually we need it i.e. after reading of that register.

Signed-off-by: Laxman Dewangan <ldewangan@nvidia.com>
Reviewed-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>