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/common/spl/Kconfig b/common/spl/Kconfig
index 25cd18a..00332cf 100644
--- a/common/spl/Kconfig
+++ b/common/spl/Kconfig
@@ -857,7 +857,7 @@
 	  allows DRAM to be set up before loading U-Boot into that DRAM,
 	  where it can run.
 
-config SPL_MTD_SUPPORT
+config SPL_MTD
 	bool "Support MTD drivers"
 	help
 	  Enable support for MTD (Memory Technology Device) within SPL. MTD