ARM: select HAVE_SMP for V7 multi-platform

All V7 platforms can run SMP kernels, so make CONFIG_SMP visible for 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>