PCI: imx6: Simplify switch-case logic by introducing init_phy() callback

Instead of using the switch case statement to initialize the PHY handled by
this driver itself, let's introduce a new callback init_phy() and define it
for platforms that require it.

Link: https://lore.kernel.org/r/20240220161924.3871774-7-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>