arm64: dts: allwinner: pinebook: Fix backlight regulator

The output from the backlight regulator is labeled as "VBKLT" in the
schematic. Using the equation and resistor values from the schematic,
the output is approximately 18V, not 3.3V. Since the regulator in use
(SS6640STR) is a boost regulator powered by PS (battery or AC input),
which are both >3.3V, the output could not be 3.3V anyway.

Signed-off-by: Samuel Holland <samuel@sholland.org>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>