mtd: onenand: select MTD_PARTITIONS

All of the onenand drivers depend on mtd partition support being compiled
in, so just select it. Fixes up build breakage:

drivers/built-in.o: In function `generic_onenand_remove':
generic.c:(.devexit.text+0x80): undefined reference to `del_mtd_partitions'

Reported-by: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>
Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>