board/radxa/rock5b: do not let the kernel mount the rootfs as rw

Do not let the kernel mount the rootfs as rw.
There is no need for this, and init will remount the rootfs as rw anyway.

Signed-off-by: Niklas Cassel <cassel@kernel.org>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>

board/radxa/rock5b/extlinux.conf

@@ -1,4 +1,4 @@
 label Radxa Rock 5b Linux
   kernel /boot/Image
   devicetree /boot/rk3588-rock-5b.dtb
-  append root=/dev/mmcblk1p1 rw earlycon rootwait
+  append root=/dev/mmcblk1p1 earlycon rootwait