davinci: omapl138_lck: remove obsolete define

NAND_MAX_CHIPS is not used anymore and has been replaced by
CONFIG_SYS_MAX_NAND_DEVICE. There is no need to keep the former
define.

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