arm: highbank: Limit FDT and initrd load addresses

So far on Highbank/Midway machines U-Boot only ever uses 512MB of DRAM,
even though the machines have typically 4GB and 8GB, respectively.
That means that so far we didn't need an extra limit for placing the DTB
and initrd, as the 512MB are lower than the kernel's limit ("lowmem",
typically 768MB).

With U-Boot now needing to learn about the actual memory size (to
correctly populate the EFI memory map), it might relocate fdt and initrd
to the end of DRAM, which is out of reach of the kernel.

So add limiting values to the fdt_high and initrd_high environment
variables, to prevent U-Boot from using too high addresses.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Reviewed-by: Tom Rini <trini@konsulko.com>