ath79: port cybertan_part from ar71xx

ath79: port cybertan_part from ar71xx

This patch ports the cybertan_part code from ar71xx and converts the
driver to a DT-supported mtd parser. As a result, it will no longer
add the u-boot, nvram and art partitions, which were never part of
the special Cybertan header.

Instead these partitions have to be specified in the DT, which has the
upside of making it possible to add properties (i.e.: read-only), labels
and references to these important partitions.

Signed-off-by: Christian Lamparter <chunkeey@gmail.com>