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>

include/configs/udoo_neo.h

@@ -34,6 +34,7 @@
 	"initrd_high=0xffffffff\0" \
 	"fdtfile=undefined\0" \
 	"fdt_addr=0x83000000\0" \
+	"fdt_addr_r=0x83000000\0" \
 	"ip_dyn=yes\0" \
 	"mmcdev=0\0" \
 	"mmcrootfstype=ext4\0" \