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>