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/include/smbios.h

@@ -40,6 +40,7 @@ const char *smbios_system_sku(void);
 
 unsigned int smbios_cpu_get_max_speed_mhz(void);
 unsigned int smbios_cpu_get_current_speed_mhz(void);
+unsigned int smbios_cpu_get_voltage(void);
 
 const char *smbios_mainboard_manufacturer(void);
 const char *smbios_mainboard_product_name(void);