Blackfin arch: let PCI depend on BROKEN

AFAIR there exists blackfin hardware with PCI support, but the support
currently in the kernel fails to build starting with:

...
  CC      drivers/pci/probe.o
probe.c: In function 'pci_scan_slot':
probe.c:1056: error: implicit declaration of function 'pcibios_scan_all_fns'
make[3]: *** [drivers/pci/probe.o] Error 1

Signed-off-by: Adrian Bunk <bunk@kernel.org>
Signed-off-by: Bryan Wu <cooloney@kernel.org>