arm: mvebu: Add support for MV78260

This patch adds support for the dual core Armada XP variant, the
MV78260. It has some minor differences to the 4-core MV78460,
e.g. only 12 serdes lanes.

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