arm: mvebu: turris_omnia: Fix MTD partitions order for Linux

Linux enumerates MTD partitions in DTB order, while the fdt_add_subnode()
function puts a new subnode at the beginning.

To fix this, put MTD partitions into DTB in reverse order.

Fixes: 92f36c8e ("arm: mvebu: turris_omnia: fixup MTD partitions in Linux' DTB")
Signed-off-by: Pali Rohár <pali@kernel.org>
Reviewed-by: Marek Behún <marek.behun@nic.cz>
Reviewed-by: Stefan Roese <sr@denx.de>