arm64: dts: qcom: pmk8550: fix PON compatible

The right compatible is qcom,pmk8350-pon, it matches the
reg resources associated to the node and the subnodes compatible
properties.

Fixes: e9c0a4e4 ("arm64: dts: qcom: Add PMK8550 pmic dtsi")
Signed-off-by: Neil Armstrong <neil.armstrong@linaro.org>
Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Signed-off-by: Bjorn Andersson <andersson@kernel.org>
Link: https://lore.kernel.org/r/20230207-topic-sm8550-upstream-sm8550-dt-fix-v1-2-698d132ab285@linaro.org