MAINTAINERS: Add entry for uDPU board

This adds an entry maintainer for the 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).

[gregory: maintainer part extract from initial "arm64: dts: marvell: Add
device tree for uDPU board" pacth]
Signed-off-by: Vladimir Vid <vladimir.vid@sartura.hr>
Signed-off-by: Gregory CLEMENT <gregory.clement@bootlin.com>