video: backlight: Parse PWM polarity cell

This patch enables the reading of the polarity cell from a PWM
phandle and calls pwm_set_invert().

Not all platforms have polarity cell, so skip if it's not pressent.

Signed-off-by: Stefan Mavrodiev <stefan@olimex.com>
Reviewed-by: Anatolij Gustschin <agust@denx.de>