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/dra7xx_hs_evm_defconfig b/configs/dra7xx_hs_evm_defconfig
index c71f2dd..60cb5b9 100644
--- a/configs/dra7xx_hs_evm_defconfig
+++ b/configs/dra7xx_hs_evm_defconfig
@@ -36,6 +36,8 @@
 CONFIG_SPL_DMA=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_RAM_SUPPORT=y
 CONFIG_SPL_SPI_LOAD=y
 CONFIG_SPL_USB_GADGET=y