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_hs_evm_defconfig b/configs/am335x_hs_evm_defconfig
index 1bbc71f..3a88a6b 100644
--- a/configs/am335x_hs_evm_defconfig
+++ b/configs/am335x_hs_evm_defconfig
@@ -22,6 +22,8 @@
 # CONFIG_SPL_FS_EXT4 is not set
 CONFIG_SPL_MTD_SUPPORT=y
 # CONFIG_SPL_NAND_SUPPORT is not set
+CONFIG_SPL_NAND_DRIVERS=y
+CONFIG_SPL_NAND_ECC=y
 # CONFIG_SPL_YMODEM_SUPPORT is not set
 # CONFIG_CMD_FLASH is not set
 CONFIG_CMD_NAND=y