RSA depends on DM

Discovered while experimenting with signature checking on vexpress
which doesn't typically use DM.
Acked-by: Simon Glass <sjg@chromium.org>
diff --git a/Kconfig b/Kconfig
index 9af31e3..09e2bbe 100644
--- a/Kconfig
+++ b/Kconfig
@@ -118,6 +118,7 @@
 config FIT_SIGNATURE
 	bool "Enable signature verification of FIT uImages"
 	depends on FIT
+	depends on DM
 	select RSA
 	help
 	  This option enables signature verification of FIT uImages,