NAND: remove NAND_MAX_CHIPS definitions

This change follows the change by Wolfgang Grandegger (commit 6c869637),
which allows to remove useless NAND_MAX_CHIPS definitions in board config
files.

Signed-off-by: Vladimir Zapolskiy <vz@mleia.com>
Cc: Wolfgang Grandegger <wg@grandegger.com>
Cc: Scott Wood <scottwood@freescale.com>
Signed-off-by: Scott Wood <scottwood@freescale.com>