Currently we let U-Boot find a spot at the end of DRAM at runtime, this
forces us to build an OPTEE image based on the size of DRAM for an EVM.
Add a default address that works across all current AM57xx EVMs.

Signed-off-by: Andrew F. Davis <afd@ti.com>
Reviewed-by: Tom Rini <trini@konsulko.com>