MIPS: BCM47XX Avoid compile error with MIPS allnoconfig

Currently MIPS allnoconfig with CONFIG_BCM47XX=y fails to compile due to
neither BCM47XX_BCMA nor BCM47XX_SSB being selected. This leads the
enumeration in arch/mips/include/asm/mach-bcm47xx/bcm47xx.h to be empty,
and compilation fails:

In file included from arch/mips/bcm47xx/irq.c:32:0:
./arch/mips/include/asm/mach-bcm47xx/bcm47xx.h:34:1: error: expected
identifier before '}' token
 };
 ^
make[2]: *** [scripts/Makefile.build:314: arch/mips/bcm47xx/irq.o] Error 1

Fix this by ensuring that BCM47XX_SSB is selected if BCM47XX_BCMA is
not. This allows us to select either system or both, but not neither.
Signed-off-by: Matt Redfearn <matt.redfearn@mips.com>
Cc: James Hogan <james.hogan@mips.com>
Cc: linux-mips@linux-mips.org
Cc: linux-kernel@vger.kernel.org
Patchwork: https://patchwork.linux-mips.org/patch/17703/Signed-off-by: Ralf Baechle <ralf@linux-mips.org>

arch/mips/Kconfig

@@ -259,6 +259,7 @@ config BCM47XX
 	select LEDS_GPIO_REGISTER
 	select BCM47XX_NVRAM
 	select BCM47XX_SPROM
+	select BCM47XX_SSB if !BCM47XX_BCMA
 	help
 	 Support for BCM47XX based boards