security/intel/stm: Use depends on ENABLE_VMX

The STM is a part of the core VTx and using ENABLE_VMX will make the
STM option available for any configuration that has an Intel
processor that supports VTx.

Signed-off-by: Eugene D. Myers <edmyers@tycho.nsa.gov>
Change-Id: I57ff82754e6c692c8722d41f812e35940346888a
Reviewed-on: https://review.coreboot.org/c/coreboot/+/38852
Reviewed-by: Nico Huber <nico.h@gmx.de>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

src/security/intel/stm/Kconfig

@@ -1,12 +1,8 @@
-
-config PLATFORM_SUPPORTS_STM
-	bool
-	depends on SMM_TSEG
-
 config STM
 	bool "Enable STM"
 	default n
-	depends on PLATFORM_SUPPORTS_STM
+	depends on ENABLE_VMX
+	depends on SMM_TSEG
 	select USE_BLOBS
 
 	help

src/soc/intel/skylake/Kconfig

@@ -44,7 +44,6 @@ 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