So far U-Boot was hard coding a (surely sufficient) memory size of 512
MB, even though all machines out there have at least 4GB of DRAM.
Since U-Boot uses its memory knowledge to populate the EFI memory map,
we are missing out here, at best losing everything beyond 4GB on Midway
boxes (which typically come with 8GB of DRAM).

Since the management processor populated the DT memory node already with
the detected DRAM size and configuration, we use that to populate
U-Boot's memory bank information, which is the base for the UEFI memory
map.
This finally allows us to get rid of the NR_DRAM_BANKS=0 hack, that we
had in place to avoid U-Boot messing up the DT memory node before
loading the kernel.

Also, to cover the whole of memory, we need to enable PHYS_64BIT.
Signed-off-by: NAndre Przywara <andre.przywara@arm.com>