arm64: dts: qcom: apq8016-sbc: Limit MPP4 high state to 1.8V

96Boards specs require all GPIO signals to be at 1.8V.
Limit MPP4, which is PIN28 on J8, to 1.8V(L5).

Signed-off-by: Ivan T. Ivanov <ivan.ivanov@linaro.org>