PCI: iproc: Rename PCI_EXP_CAP to IPROC_PCI_EXP_CAP

PCI_EXP_CAP is an iProc-specific value, so rename it to IPROC_PCI_EXP_CAP
to make it obvious that it's not related to the generic values like
PCI_EXP_RTCTL, etc.  No functional change intended.

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