ath79: ecb1750: additional dts fixes

This patch fixes following missing bits:

 - add missing 'compatible' property on firmware partition
 - set vendor partition 'userconfig' read-only

Fixes: 30dcbc741d ("ath79: add support for EnGenius ECB1750")
Signed-off-by: Sven Friedmann <sf.openwrt@okay.ms>
Signed-off-by: Petr Štetiar <ynezz@true.cz>

target/linux/ath79/dts/qca9558_engenius_ecb1750.dts

@@ -93,6 +93,7 @@
 			};
 
 			partition@50000 {
+				compatible = "denx,uimage";
 				label = "firmware";
 				reg = <0x050000 0xf50000>;
 			};
@@ -100,6 +101,7 @@
 			partition@fa0000 {
 				label = "userconfig";
 				reg = <0xfa0000 0x050000>;
+				read-only;
 			};
 
 			art: partition@ff0000 {