mediatek: mt7988: add missing clock for PCIe ports

mediatek: mt7988: add missing clock for PCIe ports

Add missing CLK_TOP_PEXTP_Px_SEL clock for each of the 4 PCIe interfaces
of the MT7988 SoC. Without that clock PCIe doesn't work reliable.

Signed-off-by: Daniel Golle <daniel@makrotopia.org>