mvebu: mark all mtd partitions on GL.iNet GL-MV1000 read-only

On this device, two of the three defined MTD partitions are
automatically set to read-only, since they do not end at an
erase/write block boundary.

In particular, the only partition remaining writable is the
one holding the u-boot bootloader.

Mark all of the partitions read-only, at least until a better
understanding of why the layout has been laid out this way is
gained.

Signed-off-by: Enrico Mioso <mrkiko.rs@gmail.com>

target/linux/mvebu/files/arch/arm64/boot/dts/marvell/armada-3720-gl-mv1000.dts

@@ -91,16 +91,19 @@
 			partition@0 {
 				label = "u-boot";
 				reg = <0 0xf0000>;
+				read-only;
 			};
 
 			partition@f0000 {
 				label = "u-boot-env";
 				reg = <0Xf0000 0x8000>;
+				read-only;
 			};
 
 			factory: partition@f8000 {
 				label = "factory";
 				reg = <0xf8000 0x8000>;
+				read-only;
 			};
 		};
 	};