ARM: dts: sun8i: Add pwm-backlight device for A23/A33 Q8 format tablets

The LCD backlight on the A23/A33 Q8 format tablets is enabled
with a GPIO controlled regulator, and brightness controlled with
the SoC's PWM controller.

The backlight is powered from the AXP223 PMIC's DC1SW output,
which is not supported yet. A proper bootloader is required
to enable it.

The brightness levels are arbitrary. The FEX files do not have
such information. As such, actual brightness levels may differ
from device to device.

Signed-off-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>