soc/medaitek: add prompt string to config MTK_DFD

Add prompt string to allow selecting MTK_DFD manually.

TEST=Select and enable MTK_DFD then successfully built firmware images.
BUG=b:207450135

Signed-off-by: Rex-BC Chen <rex-bc.chen@mediatek.com>
Change-Id: Ied711321efa592cf1bf7b318fe4d0aa155c15c70
Reviewed-on: https://review.coreboot.org/c/coreboot/+/59621
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Yu-Ping Wu <yupingso@google.com>
diff --git a/src/soc/mediatek/common/Kconfig b/src/soc/mediatek/common/Kconfig
index b581ffb..7d1e455 100644
--- a/src/soc/mediatek/common/Kconfig
+++ b/src/soc/mediatek/common/Kconfig
@@ -36,9 +36,10 @@
 	  This option enables four channel configuration for DPM.
 
 config MTK_DFD
-	bool
-	default n
+	bool "Enable MediaTek DFD (Design For Debug) settings"
 	help
-	  This option enables DFD (Design for Debug) settings.
+	  DFD (Design for Debug) is a debugging tool, which scans flip-flops
+	  and dumps to internal RAM on the WDT reset. We reserve 1MB on DRAM
+	  to store logs of DFD.
 
 endif