arm64: dts: allwinner: a64: teres-i: enable backlight

Enable pwm and add a pretty standard backlight node.

The regulator is always on, but we include it anyway, because it is
required by the binding document.

Signed-off-by: Harald Geyer <harald@ccbib.org>
Signed-off-by: Maxime Ripard <maxime.ripard@bootlin.com>