MIPS: BCM47xx: Enable SSB prerequisite SSB_DRIVER_PCICORE.

Split of from Arend's patch.

Signed-off-by: Arend van Spriel <arend@broadcom.com>
Cc: Sergei Shtylyov <sshtylyov@mvista.com>
Cc: linux-mips@linux-mips.org
Cc: Hauke Mehrtens <hauke@hauke-m.de>
Patchwork: https://patchwork.linux-mips.org/patch/4759/


Signed-off-by: Ralf Baechle <ralf@linux-mips.org>