dt-bindings: serial: Add Milbeaut serial driver description

Add DT bindings document for Milbeaut serial driver.

Signed-off-by: Sugaya Taichi <sugaya.taichi@socionext.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>