PCI: imx6: Add external reference clock input mode support

PCI: imx6: Add external reference clock input mode support

i.MX95 PCIes have two reference clock inputs: one from internal PLL, the
other from off chip crystal oscillator. The "extref" clock refers to a
reference clock from an external crystal oscillator.

Add external reference clock input mode support 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>
Link: https://patch.msgid.link/20251211064821.2707001-4-hongxing.zhu@nxp.com