mtd: nand: omap2: Add check for old elm binding

commit c9711ec5 ("mtd: nand: omap: Clean up device tree support")
removes the check for the old elm phandle binding.
Add it again to keep backward compatibility.

Fixes: commit c9711ec5 ("mtd: nand: omap: Clean up device tree support")
Signed-off-by: Teresa Remmet <t.remmet@phytec.de>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>