ipq806x: replace linux,part-probe with a proper partitions subnode

ipq806x: replace linux,part-probe with a proper partitions subnode

This solution is more upstream compatible as it only requires specifying
of_match_table in the parser code and doesn't depend on linux,part-probe
which is solution made generic by a LEDE downstream patch that can't be
upstreamed.

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Tested-by: Stefan Lippers-Hollmann <s.l-h@gmx.de>
Acked-by: John Crispin <john@phrozen.org>