mtd: omap: fix mtd devices not showing up

Since commit 6d178ef2

 ("mtd: nand: Move ELM driver and rename as
omap_elm"), I don't have any mtd devices present on my am335x. This
changes the link order of the omap_elm and omap2 objects, causing them
to probe in the wrong order.

To fix this, make elm_config defer probing until the omap_elm driver is
actually loaded.

Signed-off-by: Frans Klaver <frans.klaver@xsens.com>
Acked-by: Roger Quadros <rogerq@ti.com>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>