vexpress64: pick DRAM size from DT

vexpress64: pick DRAM size from DT

So far the DRAM size for both the Juno and the FVP model were hardcoded
in our config header file. For the Juno this is fine, as all models have
8 GiB of DRAM, but the DRAM size can be configured on the model command
line.

Drop the fixed DRAM size setup, instead look up the size in the device
tree, that we now have for every board. This allows a user to inject
a DT with the proper size, and be able to use the full amount of DRAM.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>