mfd: twl4030-power: Start transition to DT

Support for loading twl4030-power module via devicetree.
For now, when booting with a DT, only the poweroff callback
feature is supported through the ti,use_poweroff property.

Signed-off-by: Florian Vaussard <florian.vaussard@epfl.ch>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>