Convert CONFIG_NAND_LPC32XX_SLC to Kconfig

This converts the following to Kconfig:
   CONFIG_NAND_LPC32XX_SLC

Signed-off-by: Adam Ford <aford173@gmail.com>
diff --git a/configs/devkit3250_defconfig b/configs/devkit3250_defconfig
index 14c24ce..8a37122 100644
--- a/configs/devkit3250_defconfig
+++ b/configs/devkit3250_defconfig
@@ -34,6 +34,7 @@
 # CONFIG_MMC is not set
 CONFIG_MTD_NOR_FLASH=y
 CONFIG_NAND=y
+CONFIG_NAND_LPC32XX_SLC=y
 CONFIG_SPL_NAND_SIMPLE=y
 CONFIG_PHYLIB=y
 CONFIG_PHY_ADDR_ENABLE=y