Kconfig: make TPL_DM_SERIAL depend on TPL_DM

This missing dependency seems like an oversight, since all other
TPL_DM_* options have it.
Signed-off-by: NThomas Hebb <tommyhebb@gmail.com>

drivers/serial/Kconfig

@@ -145,7 +145,7 @@ config SPL_DM_SERIAL
 
 config TPL_DM_SERIAL
 	bool "Enable Driver Model for serial drivers in TPL"
-	depends on DM_SERIAL
+	depends on DM_SERIAL && TPL_DM
 	default y if TPL && DM_SERIAL
 	help
 	  Enable driver model for serial in TPL. This replaces