Add buffer management entries to armada-38x.dtsi

Hardware buffer management entries are not used yet by FreeBSD.
They were added for compliance with Linux Armada 38x device tree
representation and will be used in future network support.

Submitted by: Bartosz Szczepanek <bsz@semihalf.com>
Obtained from: Semihalf
Sponsored by: Stormshield
Differential revision: https://reviews.freebsd.org/D8179