dt-bindings: sram: milbeaut: Add binding for Milbeaut smp-sram

The Milbeaut M10V SoC needs a part of sram for smp, so this adds the
M10V sram compatible and 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>