mtd: Rename SPL_MTD_SUPPORT to SPL_MTD

Rename SPL_MTD_SUPPORT to SPL_MTD in order to match MTD. This allows using
CONFIG_IS_ENABLED to test for MTD support.

Signed-off-by: Sean Anderson <seanga2@gmail.com>
diff --git a/configs/am43xx_evm_defconfig b/configs/am43xx_evm_defconfig
index 5f2356b..d5ce299 100644
--- a/configs/am43xx_evm_defconfig
+++ b/configs/am43xx_evm_defconfig
@@ -22,7 +22,7 @@
 CONFIG_SPL_SYS_MALLOC_SIZE=0x800000
 CONFIG_SPL_ETH=y
 CONFIG_SPL_FS_LOAD_PAYLOAD_NAME="u-boot.img"
-CONFIG_SPL_MTD_SUPPORT=y
+CONFIG_SPL_MTD=y
 CONFIG_SPL_NAND_DRIVERS=y
 CONFIG_SPL_NAND_ECC=y
 CONFIG_SPL_NAND_BASE=y