Convert CONFIG_SPL_NAND_DRIVERS et al to Kconfig

This converts the following to Kconfig:
   CONFIG_SPL_NAND_DRIVERS
   CONFIG_SPL_NAND_ECC
   CONFIG_SPL_NAND_SIMPLE

Signed-off-by: Adam Ford <aford173@gmail.com>
diff --git a/configs/am335x_evm_defconfig b/configs/am335x_evm_defconfig
index 1968f2f..ea9f1df 100644
--- a/configs/am335x_evm_defconfig
+++ b/configs/am335x_evm_defconfig
@@ -18,6 +18,8 @@
 # CONFIG_SPL_FS_EXT4 is not set
 CONFIG_SPL_MTD_SUPPORT=y
 CONFIG_SPL_MUSB_NEW_SUPPORT=y
+CONFIG_SPL_NAND_DRIVERS=y
+CONFIG_SPL_NAND_ECC=y
 CONFIG_SPL_NET_SUPPORT=y
 CONFIG_SPL_NET_VCI_STRING="AM335x U-Boot SPL"
 CONFIG_SPL_OS_BOOT=y