PCI: brcmstb: Allow building for BMIPS_GENERIC

BMIPS_GENERIC denotes support for the MIPS-based Broadcom STB platforms
which this driver can support.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>