arm: mvebu: Make ECC support configurable on Armada XP

Currently, ECC support is enabled for all Armada XP boards. So the
DDR3 driver tries to configure the controller with ECC support, even
on boards without ECC. This patch makes this ECC optional which now
can be configured on a board-per-board basis.

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