armv8: ls2080: Update bootcmd env variable

Before Linux boot DPL must be deployed to DPAA2 interface.
So update bootcmd env variable to make sure DPL deployment before
Linux boot.

Signed-off-by: Prabhakar Kushwaha <prabhakar.kushwaha@nxp.com>
Reviewed-by: York Sun <york.sun@nxp.com>