arm64: dts: qcom: pm8916: Add PON node

Add PON and pwrkey as child nodes for PON device. Also
add additional properties for pwrkey i.e., linux,code

Signed-off-by: Vinod Koul <vkoul@kernel.org>
Reviewed-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Signed-off-by: Andy Gross <andy.gross@linaro.org>