regulator: add pbias regulator support

pbias register controls internal power supply to sd card i/o pads
in most OMAPs (OMAP2-5, DRA7).
Control bits for selecting voltage level and
enabling/disabling are in the same PBIAS register.

Signed-off-by: Balaji T K <balajitk@ti.com>
Acked-by: Tony Lindgren <tony@atomide.com>
Acked-by: Mark Brown <broonie@linaro.org>
Tested-by: Florian Vaussard <florian.vaussard@epfl.ch>
Tested-by: Stefan Roese <sr@denx.de>
Signed-off-by: Chris Ball <chris@printf.net>