Convert CONFIG_SPL_NAND_BASE et al to Kconfig

This converts the following to Kconfig:
   CONFIG_SPL_NAND_BASE
   CONFIG_SPL_NAND_IDENT

Signed-off-by: Adam Ford <aford173@gmail.com>
diff --git a/configs/ti816x_evm_defconfig b/configs/ti816x_evm_defconfig
index e436175..26172a7 100644
--- a/configs/ti816x_evm_defconfig
+++ b/configs/ti816x_evm_defconfig
@@ -30,6 +30,7 @@
 CONFIG_SPL_NAND_SUPPORT=y
 CONFIG_SPL_NAND_DRIVERS=y
 CONFIG_SPL_NAND_ECC=y
+CONFIG_SPL_NAND_BASE=y
 CONFIG_CMD_ASKENV=y
 # CONFIG_CMD_FLASH is not set
 CONFIG_CMD_GPIO=y