PCI: xgene: Define XGENE_PCI_EXP_CAP and use generic PCI_EXP_RTCTL offset

Apparently the PCIe capability is at address 0x40 in config space of X-Gene
v1 Root Ports.  Add a definition of that and use the generic PCI_EXP_RTCTL
offset into the capability.  No functional change intended.

Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>