arm: mediatek: remove wmcpu-reserved@50000000 node from mt7987 dts

The reserved-memory node 'wmcpu-reserved@50000000' only applies to
linux kernel and is useless in u-boot.
Remove it in *-u-boot.dtsi to make this memory region usable.

Fixes: 2d6962e0618 (arm: mediatek: add support for MediaTek MT7987 SoC)
diff --git a/arch/arm/dts/mt7987a-u-boot.dtsi b/arch/arm/dts/mt7987a-u-boot.dtsi
index ec0a638..a1cb42a 100644
--- a/arch/arm/dts/mt7987a-u-boot.dtsi
+++ b/arch/arm/dts/mt7987a-u-boot.dtsi
@@ -24,6 +24,10 @@
 			mediatek,hwver = <&hwver>;
 		};
 	};
+
+	reserved-memory {
+		/delete-node/ wmcpu-reserved@50000000;
+	};
 };
 
 &i2c0 {