PCI: imx6: Simplify PHY handling by using IMX6_PCIE_FLAG_HAS_PHYDRV

Since some i.MX platforms make use of a separate PHY driver, use
IMX6_PCIE_FLAG_HAS_PHYDRV flag to identify them and get the reference to
PHY from DT to simplify the code.

Link: https://lore.kernel.org/r/20240220161924.3871774-3-Frank.Li@nxp.com


Signed-off-by: Frank Li <Frank.Li@nxp.com>
Signed-off-by: Lorenzo Pieralisi <lpieralisi@kernel.org>
Reviewed-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>