Convert CONFIG_SYS_MAX_NAND_DEVICE to Kconfig

This converts the following to Kconfig:
   CONFIG_SYS_MAX_NAND_DEVICE

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 d6e3eeb..a7196d5 100644
--- a/drivers/mtd/nand/raw/Kconfig
+++ b/drivers/mtd/nand/raw/Kconfig
@@ -26,6 +26,10 @@
 config TPL_NAND_INIT
 	bool
 
+config SYS_MAX_NAND_DEVICE
+	int "Maximum number of NAND devices to support"
+	default 1
+
 config SYS_NAND_DRIVER_ECC_LAYOUT
 	bool "Omit standard ECC layouts to save space"
 	help