ipq806x: add missing enclosing partitions block for TP-Link C2600

The partitions that have compatible property set are skipped by mtd if
they are not contained inside a partitions node and this breaks
fetching MAC address from "default-mac" partition.

Fix this by defining all the partitions inside partitions node with
compatible = "fixed-partitions" as nvmem requires the standard
partitions scheme to work correctly.

Fixes: FS#3945
Fixes: cd36d716 ("ipq806x/dts: Add Archer C2600 DTS")
Fixes: 0458a899 ("ipq806x: convert mtd-mac-address to nvmem
implementation")

Signed-off-by: Filip Matijević <filip.matijevic.pz@gmail.com>
Reviewed-by: Ansuel Smith <ansuelsmth@gmail.com>
[adjust commit title/message]
Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>