davinci: omapl138_lcdk: use correct define for 16 bit NAND chips

The omapl138_lcdk header defines CONFIG_SYS_NAND_BUSWIDTH_16_BIT while
the correct name is CONFIG_SYS_NAND_BUSWIDTH_16BIT.
While renaming the only occurrence of CONFIG_SYS_NAND_BUSWIDTH_16_BIT,
let's also remove it from the config_whitelist.txt file.

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