mtd: atmel_nand: fix the warning when CONFIG_OF is not defined

This patch fix following warning:

drivers/mtd/nand/atmel_nand.c:2007: warning: 'atmel_nand_nfc_match' defined but not used

This patch add '#if defined(CONFIG_OF)' block to guard around the definition
of atmel_nand_nfc_match, in order to avoid the warning when the kernel is
configured without DT support.

Signed-off-by: Josh Wu <josh.wu@atmel.com>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>