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/cm_t43_defconfig b/configs/cm_t43_defconfig
index 50d242c..1679b74 100644
--- a/configs/cm_t43_defconfig
+++ b/configs/cm_t43_defconfig
@@ -33,6 +33,8 @@
 CONFIG_SPL_MTD_SUPPORT=y
 # CONFIG_SPL_NAND_SUPPORT is not set
 CONFIG_SPL_DM_SPI_FLASH=y
+CONFIG_SPL_NAND_DRIVERS=y
+CONFIG_SPL_NAND_ECC=y
 CONFIG_SPL_POWER_SUPPORT=y
 CONFIG_SPL_SPI_LOAD=y
 CONFIG_SYS_PROMPT="CM-T43 # "