ramips: fix compatibles in SoC dtsi

The former used compatibles aren't defined anywhere and aren't used by
the devicetree source files including them.

Signed-off-by: Mathias Kresin <dev@kresin.me>

target/linux/ramips/dts/mt7620a.dtsi

@@ -1,7 +1,7 @@
 / {
 	#address-cells = <1>;
 	#size-cells = <1>;
-	compatible = "ralink,mtk7620a-soc";
+	compatible = "ralink,mt7620a-soc";
 
 	cpus {
 		#address-cells = <1>;

target/linux/ramips/dts/mt7620n.dtsi

@@ -1,7 +1,7 @@
 / {
 	#address-cells = <1>;
 	#size-cells = <1>;
-	compatible = "ralink,mtk7620n-soc";
+	compatible = "ralink,mt7620n-soc";
 
 	cpus {
 		#address-cells = <1>;

target/linux/ramips/dts/mt7628an.dtsi

@@ -1,7 +1,7 @@
 / {
 	#address-cells = <1>;
 	#size-cells = <1>;
-	compatible = "ralink,mtk7628an-soc";
+	compatible = "mediatek,mt7628an-soc";
 
 	cpus {
 		#address-cells = <1>;