x86/PCI: Mark constants of pci_mmcfg_nvidia_mcp55() as __initconst

The constants in pci_mmcfg_nvidia_mcp55() need to be marked as __initconst
or they will remain in memory after init memory was released.

Signed-off-by: Mathias Krause <minipli@googlemail.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: Ingo Molnar <mingo@kernel.org>