kernel: mtdsplit_uimage: replace "allnet, uimage" parser

Convert users to the generic "openwrt,uimage" using device specific
"openwrt,ih-magic" properties, and remove "allnet,uimage".

Signed-off-by: Bjørn Mork <bjorn@mork.no>

target/linux/realtek/dts/rtl8382_allnet_all-sg8208m.dts

@@ -95,7 +95,8 @@
 			partition@2a0000 {
 				label = "firmware";
 				reg = <0x2a0000 0xd60000>;
-				compatible = "allnet,uimage";
+				compatible = "openwrt,uimage", "denx,uimage";
+				openwrt,ih-magic = <0x00000006>;
 			};
 		};
 	};