ARM: select MIGHT_HAVE_CACHE_L2X0 for V6 and V7 multi-platform

Many V6 and V7 platforms have an L2x0 cache, so make
CONFIG_MIGHT_HAVE_CACHE_L2X0 visible for V6 and V7 multi-platform
builds.

Signed-off-by: Rob Herring <robh@kernel.org>
Acked-by: Stephen Warren <swarren@nvidia.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>