ath79: make Engenius fakeroot partitions read-only

For:

 - ENH202 v1
 - ENS202EXT v1

These boards were committed before it was discovered
that for all Engenius boards with a "failsafe" image,
forcing the failsafe image to load next boot
can be achieved by editing the u-boot environment like:

  `fw_setenv rootfs_checksum 0`

So it's not necessary to delete a partition to boot to failsafe image.

Signed-off-by: Michael Pratt <mcpratt@pm.me>

target/linux/ath79/dts/ar7240_engenius_enh202-v1.dts

@@ -136,6 +136,7 @@
 			partition@1a0000 {
 				label = "fakeroot";
 				reg = <0x1a0000 0x10000>;
+				read-only;
 			};
 
 			fwconcat0: partition@1b0000 {

target/linux/ath79/dts/ar9341_engenius_ens202ext-v1.dts

@@ -130,6 +130,7 @@
 			partition@220000 {
 				label = "fakeroot";
 				reg = <0x220000 0x010000>;
+				read-only;
 			};
 
 			fwconcat0: partition@230000 {