mfd: Cache tps6586x register through regmap

To cache the interrupt mask register, use the regmap RB_TREE
cache-ing mechanism in place of implementing it locally.

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>