PCI: generic, thunder: Use generic ECAM API

Use functions provided by drivers/pci/ecam.h for mapping the config space
in drivers/pci/host/pci-host-common.c, and update its users to use 'struct
pci_config_window' and 'struct pci_ecam_ops'.

The changes are mostly to use 'struct pci_config_window' in place of
'struct gen_pci'.  Some of the fields of gen_pci were only used temporarily
and can be eliminated by using local variables or function arguments, these
are not carried over to struct pci_config_window.

pci-thunder-ecam.c and pci-thunder-pem.c are the only users of the
pci_host_common_probe function and the gen_pci structure; these have been
updated to use the new API as well.

The patch does not introduce any functional changes other than a very minor
one: with the new code, on 64-bit platforms, we do just a single ioremap
for the whole config space.

Signed-off-by: Jayachandran C <jchandra@broadcom.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>