PCI: layerscape: Add function pointer for exit_from_l2()

Since different SoCs require different sequences for exiting L2, let's add
a separate "exit_from_l2()" callback to handle SoC specific sequences.

Change ls_pcie_exit_from_l2() return value from void to int in order
to propagate errors. Return an error if the exit_from_l2() callback
fails in the resume flow.

Link: https://lore.kernel.org/r/20231204160829.2498703-2-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>
Acked-by: Roy Zang <Roy.Zang@nxp.com>