mmc: omap_hsmmc: use devm_regulator API

Use devm_regulator API, while at it use
devm_regulator_get_optional for optional vmmc_aux supply

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