mtd: physmap_of: fix null pointer deference when kzalloc returns null

static analysis by smatch caught the following error:

drivers/mtd/maps/physmap_of.c:135 of_get_probes()
   error: potential null dereference 'res'.  (kzalloc returns null)

Check for failed kzalloc and return -ENOMEM in of_flash_probe if
this occurs.

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>