ARM: Select V6K instead of V6 by default for multi-platform

MULTI_V6 should default to V6K as it is more optimal than V6. Any
platform which is not V6K should select CPU_V6 which will enable the
less optimal code paths.

Signed-off-by: Rob Herring <robh@kernel.org>
Cc: Anton Vorontsov <anton@enomsg.org>
Acked-by: Shawn Guo <shawn.guo@linaro.org>
Cc: Sascha Hauer <kernel@pengutronix.de>
Cc: Jamie Iles <jamie@jamieiles.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>