mmc: omap_hsmmc: adapt hsmmc to use pbias regulator

In DT case, PBAIS registers are programmed via regulator,
use regulator APIs to control PBIAS.

Signed-off-by: Balaji T K <balajitk@ti.com>
Tested-by: Florian Vaussard <florian.vaussard@epfl.ch>
Tested-by: Stefan Roese <sr@denx.de>
Signed-off-by: Chris Ball <chris@printf.net>