ARM: highbank: remove DRAM bank setup

On the highbank platform the SoC's management controller firmware
will probe the DRAM modules and populates the initial device tree with
the correct values. Therefore the memory sizes in the DT are already
correct, so remove U-Boot's DRAM bank setup so the memory node is not
"fixed up" by u-boot.

Signed-off-by: Rob Herring <robh@kernel.org>
Signed-off-by: Andre Przywara <osp@andrep.de>