mfd: twl-core: Convert to use regmap for I/O

Remove the custom code to do I/O and replace it with standard regmap calls.

Reviewed-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Acked-by: Tero Kristo <t-kristo@ti.com>
Signed-off-by: Peter Ujfalusi <peter.ujfalusi@ti.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>