ARM: BCM: put back ARCH_MULTI_V7 dependency for mobile

A recent cleanup rearranged the Kconfig file for mach-bcm and
accidentally dropped the dependency on ARCH_MULTI_V7, which
makes it possible to now build the two mobile SoC platforms
on an ARMv6-only kernel, resulting in a log of Kconfig
warnings like

warning: ARCH_BCM_MOBILE selects ARM_ERRATA_775420 which has unmet direct dependencies (CPU_V7)

and which of course cannot work on any machine.

This puts back the dependencies as before.
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Fixes: 64e74aa7 ("ARM: mach-bcm: ARCH_BCM_MOBILE: remove one level of menu from Kconfig")
Acked-by: Florian Fainelli <f.fainelli@gmail.com>
Acked-by: Scott Branden <sbranden@broadcom.com>

arch/arm/mach-bcm/Kconfig

@@ -68,7 +68,7 @@ config ARCH_BCM_MOBILE
 	  This enables support for systems based on Broadcom mobile SoCs.
 
 config ARCH_BCM_281XX
-	bool "Broadcom BCM281XX SoC family"
+	bool "Broadcom BCM281XX SoC family" if ARCH_MULTI_V7
 	select ARCH_BCM_MOBILE
 	select HAVE_SMP
 	help
@@ -77,7 +77,7 @@ config ARCH_BCM_281XX
 	  variants.
 
 config ARCH_BCM_21664
-	bool "Broadcom BCM21664 SoC family"
+	bool "Broadcom BCM21664 SoC family" if ARCH_MULTI_V7
 	select ARCH_BCM_MOBILE
 	select HAVE_SMP
 	help