ARM: dts: sun8i: Add PWM channel 0 pinmux setting for A23/A33

The PWM controller has 2 outputs, with one usable pin for each.

Add a pinmux setting for the first channel. This is often used
for backlight dimming on tablets.

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