dt-bindings: timer: Add Milbeaut M10V timer description

Add DT bindings document for Milbeaut M10V timer.

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