mpc85xx: Fix HiveAP-330 nvmem mac loader

mpc85xx: Fix HiveAP-330 nvmem mac loader

We actually need to enclose the whole section of partitions in a
`partitions { ... }` to assign it a `compatible = "fixed-partitions";
otherwise the partition referred to by `hwinfo` won't be registered
when bringing up MTD partitions, for example as per:

- <https://forum.openwrt.org/t/tp-link-c2600-missing-default-mac-mtd-partition-in-snapshot/103945/6>

- commit e2b03c16eb44 ("ipq806x: add missing enclosing partitions block for TP-Link C2600")'

Fixes: 8ec21d6bb210 ("mpc85xx: convert mtd-mac-address to nvmem implementation")
Signed-off-by: Martin Kennedy <hurricos@gmail.com>
[minor beautification]
Signed-off-by: Christian Lamparter <chunkeey@gmail.com>