bcma: Fix 'allmodconfig' and BCMA builds on MIPS targets

Mips builds with BCMA host mode enabled fail in mainline and -next
with:

In file included from include/linux/bcma/bcma.h:10:0,
                 from drivers/bcma/bcma_private.h:9,
		 from drivers/bcma/main.c:8:
include/linux/bcma/bcma_driver_pci.h:218:24: error:
	field 'pci_controller' has incomplete type

Bisect points to commit d41e6858 ("MIPS: Kconfig: Set default MIPS
system type as generic") as the culprit. Analysis shows that the commmit
changes PCI configuration and enables PCI_DRIVERS_GENERIC. This in turn
disables PCI_DRIVERS_LEGACY. 'struct pci_controller' is, however, only
defined if PCI_DRIVERS_LEGACY is enabled.

Ultimately that means that BCMA_DRIVER_PCI_HOSTMODE depends on
PCI_DRIVERS_LEGACY. Add the missing dependency.

Fixes: d41e6858 ("MIPS: Kconfig: Set default MIPS system type as ...")
Cc: Matt Redfearn <matt.redfearn@imgtec.com>
Cc: James Hogan <jhogan@kernel.org>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Reviewed-by: James Hogan <jhogan@kernel.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>