ramips: lock u-boot partition and unlock u-boot-env for miwifi-mini

Generally u-boot should keep read-only to avoid mis-overwriting and
bricking the device, but u-boot-env could be safely modified with u-boot
setenv tool.

Signed-off-by: David Yang <mmyangfl@gmail.com>

target/linux/ramips/dts/mt7620a_xiaomi_miwifi-mini.dts

@@ -71,12 +71,12 @@
 			partition@0 {
 				label = "u-boot";
 				reg = <0x0 0x30000>;
+				read-only;
 			};
 
 			partition@30000 {
 				label = "u-boot-env";
 				reg = <0x30000 0x10000>;
-				read-only;
 			};
 
 			factory: partition@40000 {