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>
diff --git a/arch/mips/dts/mt7628a.dtsi b/arch/mips/dts/mt7628a.dtsi
index 6bc0722..e9241a0 100644
--- a/arch/mips/dts/mt7628a.dtsi
+++ b/arch/mips/dts/mt7628a.dtsi
@@ -124,7 +124,7 @@
 		};
 
 		uart0: uartlite@c00 {
-			compatible = "ns16550a";
+			compatible = "mediatek,hsuart", "ns16550a";
 			reg = <0xc00 0x100>;
 
 			clock-frequency = <40000000>;
@@ -139,7 +139,7 @@
 		};
 
 		uart1: uart1@d00 {
-			compatible = "ns16550a";
+			compatible = "mediatek,hsuart", "ns16550a";
 			reg = <0xd00 0x100>;
 
 			clock-frequency = <40000000>;
@@ -154,7 +154,7 @@
 		};
 
 		uart2: uart2@e00 {
-			compatible = "ns16550a";
+			compatible = "mediatek,hsuart", "ns16550a";
 			reg = <0xe00 0x100>;
 
 			clock-frequency = <40000000>;