xilinx: Add sd boot command script for reference

This patch adds sdboot command script for reference.
This can be converetd into uboot script using mkimage and
use for booting.

Signed-off-by: Siva Durga Prasad Paladugu <siva.durga.paladugu@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>