regulator: twl: adapt twl-regulator driver to dt

Modify the twl regulator driver to extract the regulator_init_data from
device tree when passed, instead of getting it through platform_data
structures (on non-DT builds)

Also add documentation for TWL regulator specific bindings.

Signed-off-by: Rajendra Nayak <rnayak@ti.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>