PCI: iproc: Add PCI_DOMAIN dependency to PCI Kconfig

2+ PCI devices fail to be discovered due to each bus having the same PCI
domain.  This is because the domain defined in the device tree file is not
being added due to PCI_DOMAIN not being enabled.  So, every PCI bus has a
domain of zero.  When PCI_DOMAIN is selected by the Kconfig, it picks up
the domain defined in the device tree file and everything works as
expected.

Since both PCIE_IPROC_PLATFORM and PCIE_IPROC_BCMA need PCI_DOMAIN, move
it to PCIE_IPROC so it will be automatically selected for both.

Signed-off-by: Jon Mason <jonmason@broadcom.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>