ath79: add Archer C58/C59 partition compatible

This commit adds firmware partition compatible for the
TP-Link Archer C58 v1 and Archer C59 v1.

Since a22311e this allows to select the correct mtdsplit
parser instead of trying all available ones one by one.

Signed-off-by: David Bauer <mail@david-bauer.net>

target/linux/ath79/dts/qca9561_tplink_archer-c58-v1.dts

@@ -93,6 +93,7 @@
 			};
 
 			partition@20000 {
+				compatible = "denx,uimage";
 				label = "firmware";
 				reg = <0x020000 0x7c0000>;
 			};

target/linux/ath79/dts/qca9561_tplink_archer-c59-v1.dts

@@ -116,6 +116,7 @@
 			};
 
 			partition@20000 {
+				compatible = "denx,uimage";
 				label = "firmware";
 				reg = <0x020000 0xe30000>;
 			};