mtd: mxc_nand: add support for multiple chips on V21 devices

Do the following to add support for up to 4 chips on V21 devices (i.MX25 and
i.MX35):

* implement .select_chip for V21
* adjust existing NFC_V1_V2_BUF_ADDR writes to take chip select into account
* unlock all chip selects at preset_v1_v2()
* scan up to 4 devices at .probe

This has been tested on i.MX25 with two attached NAND chip (on one die).

Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>