gemini: Make root filesystem writeable

The squashfs+jffs2 root filesystem only came up as read-only
and the config would not take:

[   25.600237] mount_root: Could not open mtd device: /dev/mtd4
[   25.634674] mount_root: reading rootfs_data failed
[   25.665346] mount_root: Could not open mtd device: /dev/mtd3
[   25.699802] mount_root: reading rootfs failed
[   25.726426] mount_root: mounting /dev/root

Well that is not very strange since the whole firmware partition
is made read-only in the device tree. Let's fix that by augmenting
the OpenWrt patch.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>