PCI: imx6: Add Refclk for i.MX95 PCIe

PCI: imx6: Add Refclk for i.MX95 PCIe

Add "ref" clock to enable Refclk. To avoid breaking DT backwards
compatibility, the i.MX95 "ref" clock is optional. Use
devm_clk_get_optional() to fetch i.MX95 PCIe optional clocks in driver.

If using external clock, "ref" clock should point to external reference.

If using internal clock, CREF_EN in LAST_TO_REG controls reference output,
implemented in drivers/clk/imx/clk-imx95-blk-ctl.c.

Link: https://lore.kernel.org/r/20241126075702.4099164-3-hongxing.zhu@nxp.com
Signed-off-by: Richard Zhu <hongxing.zhu@nxp.com>
Signed-off-by: Krzysztof Wilczyński <kwilczynski@kernel.org>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Frank Li <Frank.Li@nxp.com>