mfd: sec: Add register cache for interrupt mask registers

The performance of regmap-irq is improved if the interrupt mask registers
can be cached since it does read/modify/update cycles so start using the
register cache infrastructure for those registers. We should use this more
widely but I don't have a datasheet and this is a nice, conservative
starting point.

Signed-off-by: Mark Brown <broonie@linaro.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>