Enable multi chip support in the NAND layer

This patch adds support for NAND_MAX_CHIPS to the MTD NAND layer.
Multi-chips devices are displayed as shown:

  Device 0: 2x NAND 512MiB 3,3V 8-bit, sector size 128 KiB

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