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/stm32f769-disco_spl_defconfig b/configs/stm32f769-disco_spl_defconfig
index 3462203..37d22f8 100644
--- a/configs/stm32f769-disco_spl_defconfig
+++ b/configs/stm32f769-disco_spl_defconfig
@@ -33,7 +33,7 @@
 CONFIG_SPL_NO_BSS_LIMIT=y
 CONFIG_SPL_BOARD_INIT=y
 CONFIG_SPL_SYS_MALLOC_SIMPLE=y
-CONFIG_SPL_MTD_SUPPORT=y
+CONFIG_SPL_MTD=y
 CONFIG_SPL_XIP_SUPPORT=y
 CONFIG_SPL_PAYLOAD_ARGS_ADDR=0x81c0000
 CONFIG_SPL_DM_RESET=y