arm: zynq: dts: Add U-Boot device tree additions

We need to mark some device tree nodes so that they are available before
relocation. This enables driver model to find these automatically. In the
case of SPL it ensures that these nodes will be retained in SPL.

Signed-off-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>