ARM: dts: qcom: replace gcc PXO with pxo_board fixed clock

In the Linux kernel, the following vulnerability has been resolved:

ARM: dts: qcom: replace gcc PXO with pxo_board fixed clock

Replace gcc PXO phandle to pxo_board fixed clock declared in the dts.

gcc driver doesn't provide PXO_SRC as it's a fixed-clock. This cause a

kernel panic if any driver actually try to use it.