arm64: dts: qcom: sa8775p: pmic: add the power key

Add the power key node under the PON node for PMIC #0 on sa8775p.

Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>
Reviewed-by: Konrad Dybcio <konrad.dybcio@linaro.org>
Signed-off-by: Bjorn Andersson <andersson@kernel.org>
Link: https://lore.kernel.org/r/20230327125316.210812-10-brgl@bgdev.pl