serial: MediaTek: add high-speed uart driver for MediaTek SoCs

serial: MediaTek: add high-speed uart driver for MediaTek SoCs

Many SoCs from MediaTek have a high-speed uart. This UART is compatible
with the ns16550 in legacy mode. It has extra registers for high-speed
mode which can reach a maximum baudrate at 921600.

However this UART will no longer be compatible if it's in high-speed mode.
Some BootROM of MediaTek's SoCs will change the UART into high-speed mode
and the U-Boot must use this driver to initialize the UART.

Signed-off-by: Weijie Gao <weijie.gao@mediatek.com>
Tested-by: Ryder Lee <ryder.lee@mediatek.com>
Reviewed-by: Simon Glass <sjg@chromium.org>