arm: vf610: add NAND support for vf610twr

This adds NAND support for the Vybrid tower system (TWR-VF65GS10)
provided by the vf610_nfc driver. Full 16-Bit bus width is
supported. Also an aditional config vf610twr_nand is introduced
which gets the environment from NAND. However, booting U-Boot from
NAND is not yet possible due to missing boot configuration block
(BCB).

Signed-off-by: Stefan Agner <stefan@agner.ch>