The generic ELF loading code maps the kernel into low memory
by subtracting KERN_BASE. So the copyin/copyout/readin functions
are always called with low addresses. This code finds the largest
DRAM block from the U-Boot memory map and adds that base to
the addresses.
In particular, this fixes ubldr on AM3358, which has DRAM
mapped to 0x80000000 at power-on.