ARM: dts: at91: nattis: use up-to-date mtd partitions

The nattis currently relies on the cmdline to correctly configure the
MTD partition table, and the one given in the device tree is simply
ignored. Which is fortunate, since the device tree partition table is
bonkers and does not match reality.

So, in order to avoid confusion, fix the device tree partition table
to match what is expected (i.e. what is normally provided on the cmdline).
In particular, the rootfs partition should be mtd6.

Fixes: 0e432389 ("ARM: dts: at91: add devicetree for the Axentia Nattis with Natte power")
Signed-off-by: Peter Rosin <peda@axentia.se>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>