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/Kconfig

@@ -45,3 +45,4 @@ source src/cpu/intel/socket_rPGA989/Kconfig
 # Architecture specific features
 source src/cpu/intel/fit/Kconfig
 source src/cpu/intel/turbo/Kconfig
+source src/cpu/intel/common/Kconfig