zynq: Use compiled-in DTB as we do it for Microblaze

This is easier way how to include DTB to all image formats.
Linux just take correct pointer and we don't need
to care about atags.

It will also simplify handling in our scripts.

Signed-off-by: Michal Simek <monstr@monstr.eu>

zynq: boot external dtb by default

Boot the built-in dtb if external dtb address is not provided.

DTB boot order:
 * External dtb
 * built-in dtb

Signed-off-by: Jason Wu <jason.wu@petalogix.com>