mfd: Add support for TWL4030/5030 dynamic power switching

The TWL4030/5030 family of multifunction devices allows board-specific
control of the the various regulators, clock and reset lines through
'scripts' that are loaded into its memory. This allows for Dynamic Power
Switching (DPS).

Implement board-independent core support for DPS that is then used by
board-specific code to load custom DPS scripts.

Signed-off-by: Amit Kucheria <amit.kucheria@verdurent.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>