dts: mtmips: enable high-speed UART support for mt7628

All three UARTs of mt7628 are actually MediaTek's high-speed UARTs which
support baudrate up to 921600.
The high-speed UART is compatible with ns16550 when baudrate <= 115200.

Add compatible string to dtsi file so u-boot can use it when serial_mtk
driver is built in.

Reviewed-by: Stefan Roese <sr@denx.de>
Signed-off-by: Weijie Gao <weijie.gao@mediatek.com>
1 file changed