[MIPS] Kconfig fixes for BCM47XX platform

The patch below fixes two problems for Kconfig on the BCM47xx platform:

- arch/mips/bcm47xx/gpio.c uses ssb_extif_* functions. Selecting
  SSB_DRIVER_EXTIF makes sure those functions are available.
- arch/mips/pci/pci.c needs, when enabled, platform specific functions,
  which are defined when SSB_PCICORE_HOSTMODE is enabled.

Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>