intel/stm: Add platform opt-in Kconfig

Selecting STM on an arbitrary platform would likely result in a brick,
so let's hide the prompt by default.

Change-Id: I50f2106ac05c3efb7f92fccb1e6edfbf961b68b8
Signed-off-by: Nico Huber <nico.h@gmx.de>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/38764
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: <cedarhouse1@comcast.net>
Reviewed-by: Angel Pons <th3fanbus@gmail.com>

src/security/intel/stm/Kconfig

@@ -1,9 +1,12 @@
 
+config PLATFORM_SUPPORTS_STM
+	bool
+	depends on SMM_TSEG
 
 config STM
 	bool "Enable STM"
 	default n
-	depends on SMM_TSEG
+	depends on PLATFORM_SUPPORTS_STM
 	select USE_BLOBS
 
 	help

src/soc/intel/skylake/Kconfig

@@ -44,6 +44,7 @@ config CPU_SPECIFIC_OPTIONS
 	select NO_FIXED_XIP_ROM_SIZE
 	select PARALLEL_MP
 	select PARALLEL_MP_AP_WORK
+	select PLATFORM_SUPPORTS_STM
 	select PLATFORM_USES_FSP2_0
 	select REG_SCRIPT
 	select SA_ENABLE_DPR