ARM: cns3xxx: enable V6K instead of plain V6

The cns3xxx is an ARM11MPCore which has all the V6K extensions, so V6K
should be selected instead. Dropping the select will use the default for
ARCH_MULTI_V6 which is V6K.

Signed-off-by: Rob Herring <robh@kernel.org>
Cc: Anton Vorontsov <anton@enomsg.org>