regulator: twl-regulator: Add fixed LDO for V1V8, V2V1 supply

V1V8 supply most common use is to provide VIO for the system.
V2V1 supply is used on SDP4430/PandaBoards to provide 2.1V to
twl6040, and also as an input to VCXIO_IN, VDAC_IN of twl6030.

Also update the bindings documentation with the new compatible
property for these additional LDOs.

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