mfd: Commonize tps65910 regmap access through header

This change removes the read/write callback functions in favor of common
regmap accessors inside the header file. This change also makes use of
regmap_read/write for single register access which maps better onto what this
driver actually needs.

Signed-off-by: Rhyland Klein <rklein@nvidia.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>