mfd: twl6040: Use regmap for register cache

Rather then open coding a cache of the vibra control registers use the
regmap cache code.  Also cache the interrupt mask register, providing
a small performance improvement for the interrupt code.

Signed-off-by: Mark Brown <broonie@linaro.org>
Acked-by: Peter Ujfalusi <peter.ujfalusi@ti.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>