ar71xx: fix build error due to bad include

ar71xx: fix build error due to bad include

While "rawnand.h" is available in kernel 4.14,
the default for this target is kernel 4.9 in which "nand.h" should be used.

Add an extra check to include the correct file depending on kernel version

Fixes these build errors:

drivers/mtd/nand/ar934x_nfc.c:16:10: fatal error: linux/mtd/rawnand.h: No such file or directory
 #include <linux/mtd/rawnand.h>
          ^~~~~~~~~~~~~~~~~~~~~
compilation terminated.

Fixes: 318e19ba6755 ("ar71xx: add v4.14 support")

Signed-off-by: Koen Vandeputte <koen.vandeputte@ncentric.com>