ramips: let the "reserved" MTD partition be RO for R6220

The "reserved" partition should probably be read-only, just in case. Even
not knowing it's content, other devices have marked it as such, so it
seems a good idea to do so also for this device.

Signed-off-by: Enrico Mioso <mrkiko.rs@gmail.com>
CC: Mathias Kresin <dev@kresin.me>
CC: Hanqing Wong <hquu@outlook.com>

target/linux/ramips/dts/R6220.dts

@@ -117,6 +117,7 @@
 	partition@4200000 {
 		label = "reserved";
 		reg = <0x4200000 0x3c00000>;
+		read-only;
 	};
 };