ath79: specify "firmware" partition format for I-O DATA devices

Specify firmware partition format (denx,uimage) by compatible string
for I-O DATA devices.

affected devices (&run tested)

- WN-AC1167DGR
- WN-AC1600DGR2

Signed-off-by: INAGAKI Hiroshi <musashino.open@gmail.com>

target/linux/ath79/dts/ar9342_iodata_etg3-r.dts

@@ -72,6 +72,7 @@
 			};
 
 			partition@50000 {
+				compatible = "denx,uimage";
 				label = "firmware";
 				reg = <0x050000 0x780000>;
 			};

target/linux/ath79/dts/qca9557_iodata_wn-ac-dgr.dtsi

@@ -137,6 +137,7 @@
 			};
 
 			partition@40000 {
+				compatible = "denx,uimage";
 				label = "firmware";
 				reg = <0x040000 0xe50000>;
 			};