PCI: mvebu: Constify mvebu_pcie_pm_ops structure

The mvebu_pcie_pm_ops structure is never modified, so declare it as const.

Signed-off-by: Jisheng Zhang <jszhang@marvell.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>