NAND: rename NAND_MAX_CHIPS to CONFIG_SYS_NAND_MAX_CHIPS

This patch renames NAND_MAX_CHIPS to CONFIG_SYS_NAND_MAX_CHIPS and
changes the default from 8 to 1 for the legacy and the new MTD
NAND layer. This allows to remove all NAND_MAX_CHIPS definitions
in the board config files because none of the boards use multi
chip support (NAND_MAX_CHIPS > 1) so far. The bamboo and the DU440
define

 #define NAND_MAX_CHIPS          CONFIG_SYS_MAX_NAND_DEVICE

but that's bogus and did not work anyhow.

Signed-off-by: Wolfgang Grandegger <wg@grandegger.com>
Signed-off-by: Scott Wood <scottwood@freescale.com>