arm64: dts: msm8996: add msmgpio label

This patch adds msmgpio label for pin and gpio controller so that
it can referenced in dedicated pins file and other board level gpios.

Signed-off-by: Srinivas Kandagatla <srinivas.kandagatla@linaro.org>
Signed-off-by: Andy Gross <andy.gross@linaro.org>