PCI: iproc: Fix BCMA dependency in Kconfig

The current iProc BCMA front-end driver can only work on ARM32 based
platforms; therefore its config option in Kconfig should be changed to
reflect that. This fixes arm64 allmodconfig build failure when compiling
the the iProc BCMA driver that contains struct pci_sys_data that is
arm32 specific

Signed-off-by: Ray Jui <rjui@broadcom.com>
Signed-off-by: Olof Johansson <olof@lixom.net>