arm64: dts: msm8916-longcheer-l8150: Add Volume buttons

Add nodes for Volume UP button connected to GPIO and Volume DOWN button,
which is handled by the pm8916 as is common with msm8916 devices.

Reviewed-by: Stephan Gerhold <stephan@gerhold.net>
Signed-off-by: Nikita Travkin <nikitos.tr@gmail.com>
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>