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/etamin_defconfig b/configs/etamin_defconfig
index 7bc3ac2..ee731f3 100644
--- a/configs/etamin_defconfig
+++ b/configs/etamin_defconfig
@@ -98,6 +98,7 @@
 CONFIG_MTD=y
 CONFIG_MTD_CONCAT=y
 CONFIG_MTD_RAW_NAND=y
+CONFIG_SYS_MAX_NAND_DEVICE=3
 CONFIG_NAND_OMAP_ECCSCHEME_BCH16_CODE_HW=y
 CONFIG_SYS_NAND_BLOCK_SIZE=0x80000
 CONFIG_SYS_NAND_ONFI_DETECTION=y