ARM: dts: imx6q-apalis: backlight pwm: Simplify inverted backlight

ARM: dts: imx6q-apalis: backlight pwm: Simplify inverted backlight

Set #pwm-cells to the default 3 to gain access to the parameter
which allows inverting the PWM signal. This is useful to specify
a backlight which has its highest brightness at 0.

With the change to use the PWM with inverted polarity the PWM signal
is inverted to how it was before this patch.
This changes the meaning of the values in the brightness-levels
property. I.e. the duty-cycle changes from x/255 to (255-x)/255.
Keeping the brightness-levels will then have a big brightness
jump from 0 to 127 duty cycle, the other 6 steps will then be
barely noticeable.

Change the brightness-levels to provide the same brightness-levels
as before.

Signed-off-by: Max Krummenacher <max.krummenacher@toradex.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>