ramips: dts: drop memory nodes

mt7621 and mt7628 now have the ability to detect memory size
automatically.
Drop memory nodes and let kernel determine memory size.

Signed-off-by: Chuanhong Guo <gch981213@gmail.com>

target/linux/ramips/dts/mt7628an_mediatek_linkit-smart-7688.dts

@@ -24,11 +24,6 @@
 		serial0 = &uart2;
 	};
 
-	memory@0 {
-		device_type = "memory";
-		reg = <0x0 0x8000000>;
-	};
-
 	bootstrap {
 		compatible = "mediatek,linkit";