simplefb: Claim and enable regulators

This claims and enables regulators listed in the simple framebuffer dt
node. This is needed so that regulators powering the display pipeline
and external hardware, described in the device node and known by the
kernel code, will remain properly enabled.

Signed-off-by: Chen-Yu Tsai <wens@csie.org>
Acked-by: Mark Brown <broonie@kernel.org>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>