ath79: add AVM EVA firmware partition compatible

This commit adds firmware partition compatible for the
AVM FRITZ!Box 4020 and AVM FRITZ!WLAN Repeater 300E.

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/ar7242_avm_fritz300e.dts

@@ -115,6 +115,7 @@
 			};
 
 			partition@20000 {
+				compatible = "avm,eva-firmware";
 				reg = <0x20000 0xee0000>;
 				label = "firmware";
 			};

target/linux/ath79/dts/qca9561_avm_fritz4020.dts

@@ -136,6 +136,7 @@
 			};
 
 			partition@1 {
+				compatible = "avm,eva-firmware";
 				label = "firmware";
 				reg = <0x020000 0xee0000>;
 			};