arm64: support running at addr other than linked to

This is required in the case where U-Boot is typically loaded and run at
a particular address, but for some reason the RAM at that location is not
available, e.g. due to memory fragmentation loading other boot binaries or
firmware, splitting an SMP complex between various different OSs without
using e.g. the EL2 second-stage page tables to hide the memory asignments,
or due to known ECC failures.

Signed-off-by: Stephen Warren <swarren@nvidia.com>