bcm47xx: make it possible to build bcm47xx without ssb.

arch/mips/Kconfig

arch/mips/bcm47xx/Kconfig

arch/mips/bcm47xx/Makefile

arch/mips/bcm47xx/gpio.c

arch/mips/bcm47xx/nvram.c

arch/mips/bcm47xx/serial.c

arch/mips/bcm47xx/setup.c

arch/mips/bcm47xx/time.c

arch/mips/include/asm/mach-bcm47xx/bcm47xx.h

arch/mips/include/asm/mach-bcm47xx/gpio.h

arch/mips/pci/pci-bcm47xx.c

drivers/watchdog/bcm47xx_wdt.c