arm64: dts: qcom: Add user LEDs on db820c

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>
(cherry picked from commit a5b359f9154f90f7863d2cce8889e47ef6405f8e)
Signed-off-by: Ramon Fried <rfried@codeaurora.org>