ARM: dts: imx6: skov: add pwm-regulator to control the panel's VCOM

Skov's i.MX6 based boards come in different flavors which have different panels
attached. For optimal contrast experience each panel type needs an individual
common voltage (VCOM) to drive its TFT backplane. The latter is generated by an
LCD bias supply IC controlled by a pwm as input signal. Introduce a pwm-
regulator to describe this hardware property and parameterize it appropriately
for the different boards.

Signed-off-by: Ulrich Ölmann <u.oelmann@pengutronix.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>