Microchip PolarFire SoC MMUART is ns16550 compatible, with some
additional registers. Create a simple MMUART model built on top
of the existing ns16550 model.

Signed-off-by: Bin Meng <bin.meng@windriver.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-Id: <1598924352-89526-6-git-send-email-bmeng.cn@gmail.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>