physmap_of: ensure versatile code is reachable

With the newly added physmap_of_versatile code, we get a build error
when physmap_of is in a module, because of_flash_probe_versatile
is not exported:

ERROR: "of_flash_probe_versatile" [drivers/mtd/maps/physmap_of.ko] undefined!

This adds the export, and changes the Makefile so that the code is
also put into a loadable module rather than built-in when physmap_of
itself is a module.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>