ARM: dts: mediatek: drop wrong syscon hifsys compatible for MT2701/7623

The syscon compatible for the hifsys node for Mediatek MT2701/MT7623 SoC
was wrongly added following the pattern of other clock node but it's
actually not needed as the register are not used by other device on the
SoC.

On top of this it's against the schema for hifsys YAML and causes a
dtbs_check warning.

Drop the "syscon" compatible to mute the warning and reflect the
compatible property described in the mediatek,mt2701-hifsys.yaml schema.

Reviewed-by: AngeloGioacchino Del Regno <[e-mail]>
Signed-off-by: Christian Marangi <[e-mail]>
Signed-off-by: AngeloGioacchino Del Regno <[e-mail]>

arch/arm/boot/dts/mediatek/mt2701.dtsi

···
 	};
 
 	hifsys: syscon@1a000000 {
-		compatible = "mediatek,mt2701-hifsys", "syscon";
+		compatible = "mediatek,mt2701-hifsys";
 		reg = <0 0x1a000000 0 0x1000>;
 		#clock-cells = <1>;
 		#reset-cells = <1>;

arch/arm/boot/dts/mediatek/mt7623.dtsi

···
 
 	hifsys: syscon@1a000000 {
 		compatible = "mediatek,mt7623-hifsys",
-			     "mediatek,mt2701-hifsys",
-			     "syscon";
+			     "mediatek,mt2701-hifsys";
 		reg = <0 0x1a000000 0 0x1000>;
 		#clock-cells = <1>;
 		#reset-cells = <1>;