mtd: nand: Introduce CONFIG_SYS_NAND_BUSWIDTH_16BIT

Introduce CONFIG_SYS_NAND_BUSWIDTH_16BIT option so that other NAND controller
drivers could use it when a 16-bit NAND is deployed.

drivers/mtd/nand/ndfc has CONFIG_SYS_NDFC_16BIT, so just rename it, so that
other NAND drivers could reuse the same symbol.

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Acked-by: Scott Wood <scottwood@freescale.com>
Reviewed-by: Benoît Thébaudeau <benoit.thebaudeau@advansee.com>
diff --git a/README b/README
index 5c5cd18..e9c3145 100644
--- a/README
+++ b/README
@@ -3755,9 +3755,12 @@
 - CONFIG_SYS_SRIOn_MEM_SIZE:
 		Size of SRIO port 'n' memory region
 
-- CONFIG_SYS_NDFC_16
-		Defined to tell the NDFC that the NAND chip is using a
-		16 bit bus.
+- CONFIG_SYS_NAND_BUSWIDTH_16BIT
+		Defined to tell the NAND controller that the NAND chip is using
+		a 16 bit bus.
+		Not all NAND drivers use this symbol.
+		Example of driver that uses it:
+		- drivers/mtd/nand/ndfc.c
 
 - CONFIG_SYS_NDFC_EBC0_CFG
 		Sets the EBC0_CFG register for the NDFC. If not defined