ARM: dts: armada-375: Update network description to match schema

Update the PP2 ethernet ports subnodes' names to match
schema enforced by the marvell,pp2.yaml contents.

Add new required properties ('reg') which contains information
about the port ID, keeping 'port-id' ones for backward
compatibility.

Signed-off-by: Marcin Wojtas <mw@semihalf.com>
Signed-off-by: David S. Miller <davem@davemloft.net>