dt-bindings: PCI: pci-imx6: Add external reference clock input

i.MX95 PCIes have two reference clock inputs: one from internal PLL.
It's wired inside chip and present as "ref" clock. It's not an optional
clock. The other from off chip crystal oscillator. The "extref" clock
refers to a reference clock from an external crystal oscillator through
the CLKIN_N/P pair PADs. It is an optional clock, relied on the board
design.

Add additional optional external reference clock input for i.MX95 PCIes.

Signed-off-by: Richard Zhu <hongxing.zhu@nxp.com>
Signed-off-by: Manivannan Sadhasivam <mani@kernel.org>
Reviewed-by: Frank Li <Frank.Li@nxp.com>
Reviewed-by: Rob Herring (Arm) <robh@kernel.org>
Link: https://patch.msgid.link/20251211064821.2707001-3-hongxing.zhu@nxp.com