Some outputs from the RTL8723CS are connected to the PL port (BT_WAKE_AP),
which runs at 1.8V. When BT_WAKE_AP is high, the PL pin this signal is
connected to is overdriven, and the whole PL port's voltage rises
somewhat. This results in changing voltage on the R_PWM pin (PL10),
which is the cause for backlight flickering very noticeably when typing
on a Bluetooth keyboard, because backlight intensity is highly sensitive
to the voltage of the R_PWM pin.

Limit the maximum WiFi/BT I/O voltage to 1.8V to avoid overdriving
the PL port pins via BT and WiFi IO port signals. WiFi and BT
functionality is unaffected by this change.

This completely stops the backlight flicker when using bluetooth.

Fixes: 91f480d4 ("arm64: dts: allwinner: Add initial support for Pine64 PinePhone")
Signed-off-by: Ondrej Jirman <megous@megous.com>
Link: https://lore.kernel.org/r/20200703194842.111845-4-megous@megous.comSigned-off-by: Maxime Ripard <maxime@cerno.tech>