udoo_neo: Add fdt_addr_r environment variable

According to doc/README.distro:
"fdt_addr_r:
Mandatory. The location in RAM where the DTB will be loaded or copied to when
processing the fdtdir/devicetreedir or fdt/devicetree options in
extlinux.conf."

So add the fdt_addr_r environment variable.

Suggested-by: Andreas Färber <afaerber@suse.de>
Signed-off-by: Breno Lima <breno.lima@nxp.com>