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/configs/CHIP_pro_defconfig b/configs/CHIP_pro_defconfig
index 2917960..5a12fbb 100644
--- a/configs/CHIP_pro_defconfig
+++ b/configs/CHIP_pro_defconfig
@@ -19,6 +19,7 @@
 # CONFIG_MMC is not set
 CONFIG_MTD=y
 CONFIG_MTD_RAW_NAND=y
+CONFIG_SYS_MAX_NAND_DEVICE=8
 CONFIG_SYS_NAND_BLOCK_SIZE=0x40000
 CONFIG_SYS_NAND_ONFI_DETECTION=y
 CONFIG_SYS_NAND_PAGE_SIZE=0x1000