ath79: u-boot partitions should be read-only in dts

Some of the new ath79 device trees were missing read-only tags on the
u-boot partitions.  This adds the tags where previously missing.

Signed-off-by: Russell Senior <russell@personaltelco.net>

target/linux/ath79/dts/ar7241_ubnt-unifi.dts

@@ -76,6 +76,7 @@
 			partition@0 {
 				label = "u-boot";
 				reg = <0x000000 0x040000>;
+				read-only;
 			};
 
 			partition@1 {

target/linux/ath79/dts/ar7241_ubnt-xm.dtsi

@@ -85,6 +85,7 @@
 			partition@0 {
 				label = "u-boot";
 				reg = <0x000000 0x040000>;
+				read-only;
 			};
 
 			partition@1 {

target/linux/ath79/dts/ar9132_tl_wr1043nd_v1.dts

@@ -102,6 +102,7 @@
 			partition@0 {
 				label = "u-boot";
 				reg = <0x000000 0x020000>;
+				read-only;
 			};
 
 			partition@1 {