dt-bindings: arm: Add SMP enable-method for Milbeaut

This adds a compatible string "socionext,milbeaut-m10v-smp"
for Milbeaut M10V to the 32 bit ARM CPU device tree binding.

Signed-off-by: Sugaya Taichi <sugaya.taichi@socionext.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>