arm: mvebu: Add armada-xp-maxbcm.dts for maxbcm board

This is needed for the upcoming ethernet DM conversion of the maxbcm
board. The configuration of the PHY is then extracted from the DT
instead of using the defines from the config header.

Signed-off-by: Stefan Roese <sr@denx.de>
Cc: Luka Perkov <luka.perkov@sartura.hr>