arm64: dts: qcom: db820c: Add user LEDs

The db820c has 4 "user LEDs", all connected to the PMI8994. The first
three are connected to the three current sinks provided by the TRILED
and the fourth is connected to MPP2.

By utilizing the DTEST bus the MPP is fed the control signal from the
fourth LPG block, providing a consistent interface to the user.

Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Reviewed-by: Dylan Van Assche <me@dylanvanassche.be>
Link: https://lore.kernel.org/r/20220505022706.1692554-5-bjorn.andersson@linaro.org