NAND: davinci: add support for NAND chips with 16 bits bus

The OMAPL138-LCD board uses a NAND chip with a 16 bits bus. Add
support into the davinci driver for 16 bit bus NAND chips.

Signed-off-by: Fabien Parent <fparent@baylibre.com>
Reviewed-by: Tom Rini <trini@konsulko.com>