arm64: dts: allwinner: add backlight regulator for Pinebook

Backlight power is controlled by PH6 GPIO, so add corresponding
regulator-fixed node for it. Otherwise backlight won't light up
if bootloader doesn't enable it.

Signed-off-by: Vasily Khoruzhick <anarsoul@gmail.com>
Signed-off-by: Maxime Ripard <maxime.ripard@bootlin.com>