fix(xilinx): update dtb when dtb address and tf-a ddr flow is used

Memory reservation in dtb will be done only when TF-A runs from
ddr and dtb load address is provided.
Otherwise prepare_dtb will simply return.
Empty definition of prepare_dtb is removed.

Signed-off-by: Amit Nagal <amit.nagal@amd.com>
Change-Id: Ie8a3ec51d60a7389831cfe6a112f722777930623
1 file changed