PCI: iproc: Allow BCMA bus driver to be built as module

Change CONFIG_PCIE_IPROC_BCMA to tristate to make it possible to build this
driver as a module.

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: Ray Jui <rjui@broadcom.com>