arm64: dts: qcom: pm8998: Use qcom,pm8998-pon binding for second gen pon

This changes pm8998 to use the new qcom,pm8998-pon compatible
string for the pon in order to support the gen2 pon
functionality properly.

Cc: Andy Gross <agross@kernel.org>
Cc: David Brown <david.brown@linaro.org>
Cc: Bjorn Andersson <bjorn.andersson@linaro.org>
Cc: Amit Pundir <amit.pundir@linaro.org>
Cc: Rob Herring <robh+dt@kernel.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Sebastian Reichel <sre@kernel.org>
Cc: linux-arm-msm@vger.kernel.org
Cc: devicetree@vger.kernel.org
Reviewed-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Signed-off-by: John Stultz <john.stultz@linaro.org>
Signed-off-by: Andy Gross <agross@kernel.org>