cpu/intel/common: Add/Use common function to set virtualization

Migrate duplicated enable_vmx() method from multiple CPUs to common
folder.  Add common virtualization option for CPUs which support it.

Note that this changes the default to enable virtualization on CPUs
that support it.

Change-Id: Ib110bed6c9f5508e3f867dcdc6f341fc50e501d1
Signed-off-by: Matt DeVillier <matt.devillier@gmail.com>
Reviewed-on: https://review.coreboot.org/17874
Reviewed-by: Nico Huber <nico.h@gmx.de>
Tested-by: build bot (Jenkins)

src/cpu/intel/model_206ax/Kconfig

@@ -20,6 +20,7 @@ config CPU_SPECIFIC_OPTIONS
 	#select AP_IN_SIPI_WAIT
 	select TSC_SYNC_MFENCE
 	select LAPIC_MONOTONIC_TIMER
+	select CPU_INTEL_COMMON
 
 config BOOTBLOCK_CPU_INIT
 	string
@@ -33,8 +34,4 @@ config SMM_TSEG_SIZE
 	hex
 	default 0x800000
 
-config ENABLE_VMX
-	bool "Enable VMX for virtualization"
-	default n
-
 endif