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/pxm2_defconfig b/configs/pxm2_defconfig
index 808cef4..690eb6b 100644
--- a/configs/pxm2_defconfig
+++ b/configs/pxm2_defconfig
@@ -31,6 +31,8 @@
 CONFIG_ARCH_MISC_INIT=y
 CONFIG_SPL_I2C_SUPPORT=y
 CONFIG_SPL_DM_SPI_FLASH=y
+CONFIG_SPL_NAND_DRIVERS=y
+CONFIG_SPL_NAND_ECC=y
 CONFIG_SPL_SPI_LOAD=y
 CONFIG_SPL_WATCHDOG_SUPPORT=y
 CONFIG_SPL_YMODEM_SUPPORT=y