Convert CONFIG_SYS_NAND_DBW_8 et al to Kconfig

This converts the following to Kconfig:
   CONFIG_SYS_NAND_DBW_8
   CONFIG_SYS_NAND_DBW_16

Note that all instances of the code check for CONFIG_SYS_NAND_DBW_16
being defined, and then "else" to CONFIG_SYS_NAND_DBW_8 whereas all of
the configs set CONFIG_SYS_NAND_DBW_8. So we introduce
CONFIG_SYS_NAND_DBW_16 as an option.

Signed-off-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/drivers/mtd/nand/raw/Kconfig b/drivers/mtd/nand/raw/Kconfig
index da2c579..48a7507 100644
--- a/drivers/mtd/nand/raw/Kconfig
+++ b/drivers/mtd/nand/raw/Kconfig
@@ -83,6 +83,18 @@
 	help
 	  Generate Programmable Multibit ECC (PMECC) header for SPL image.
 
+choice
+	prompt "NAND bus width (bits)"
+	default SYS_NAND_DBW_8
+
+config SYS_NAND_DBW_8
+	bool "NAND bus width is 8 bits"
+
+config SYS_NAND_DBW_16
+	bool "NAND bus width is 16 bits"
+
+endchoice
+
 endif
 
 config NAND_BRCMNAND