drm/omap: panel-dsi-cm: add regulator support

Add support for regulators used by panels found inside
of the Nokia N950, N9 and Motorola Droid 4.

Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.co.uk>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>