cpu/intel/common: Fill cpu voltage in SMBIOS tables

Introduce a weak function to let the platform code provide the processor
voltage in 100mV units.

Implement the function on Intel platforms using the MSR_PERF_STATUS msr.
On other platforms the processor voltage still reads as unknown.

Tested on Intel CFL. The CPU voltage is correctly advertised.

Change-Id: I31a7efcbeede50d986a1c096a4a59a316e09f825
Signed-off-by: Patrick Rudolph <patrick.rudolph@9elements.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/43904
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Angel Pons <th3fanbus@gmail.com>
Reviewed-by: Jonathan Zhang <jonzhang@fb.com>
Reviewed-by: Nico Huber <nico.h@gmx.de>

src/cpu/intel/common/Makefile.inc

@@ -1,5 +1,6 @@
 ramstage-$(CONFIG_CPU_INTEL_COMMON) += common_init.c
 ramstage-$(CONFIG_CPU_INTEL_COMMON) += hyperthreading.c
+ramstage-$(CONFIG_CPU_INTEL_COMMON_VOLTAGE) += voltage.c
 
 ifeq ($(CONFIG_CPU_INTEL_COMMON_TIMEBASE),y)
 bootblock-y += fsb.c