soc/mediatek: Add a prompt string for MEDIATEK_DRAM_SCRAMBLE

Make the default MEDIATEK_DRAM_SCRAMBLE value overridable by adding a
prompt string.

BUG=b:285474337
TEST=build pass and check scramble feature is disabled on serial build

Signed-off-by: Xi Chen <xixi.chen@mediatek.corp-partner.google.com>
Change-Id: I703ac9aa3ccc4dd9d0fef9949c6b0d49449971a4
Reviewed-on: https://review.coreboot.org/c/coreboot/+/75815
Reviewed-by: Yidi Lin <yidilin@google.com>
Reviewed-by: Yu-Ping Wu <yupingso@google.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

src/soc/mediatek/common/Kconfig

@@ -31,7 +31,7 @@ config MEDIATEK_DRAM_BLOB_FAST_INIT
 	  open-source policy.
 
 config MEDIATEK_DRAM_SCRAMBLE
-	bool
+	bool "Enable DRAM scramble feature"
 	default n
 	help
 	  This option enables DRAM data scramble, which can prevent DRAM data from