mvebu: add initial support for uDPU board

mvebu: add initial support for uDPU board

This adds initial support for micro-DPU (uDPU) board which is based on
Armada-3720 SoC.  micro-DPU is the single-port FTTdp distribution point
unit made by Methode Electronics which offers complete modularity with
replaceable SFP modules both for uplink and downlink (G.hn over
twisted-pair, G.hn over coax, 1G and 2.5G Ethernet over Cat-5e cable).

On-board features:
- 512 MiB DDR3
- 2 x 2.5G SFP via HSGMII SERDES interface to the A3720 SoC
- USB 2.0 Type-C connector
- 4GB eMMC
- ETSI TS 101548 reverse powering via twisted pair (RJ45) or coax (F Type)

uDPU is intented to run on kernel 4.19 on newer due to the SFP and hardware support.

Signed-off-by: Vladimir Vid <vladimir.vid@sartura.hr>