uboot-imx

MLK-19882 imx8qm/qxp: introduce dom0fdt_file env

Introduce dom0fdt_file env to let user could change
the dom0 dtb file.

Signed-off-by: Peng Fan <peng.fan@nxp.com>
Reviewed-by: Ye Li <ye.li@nxp.com>
(cherry picked from commit 3d99b1667943658dafd4c3f5d98bab027aab9346)