libpayload: honor TSC information under CONFIG_LP_TIMER_RDTSC

When CONFIG_LP_TIMER_RDTSC is enabled honor the TSC information
exported in the coreboot tables as the cpu_khz frequency. That
allows get_cpu_speed() not to be called which currently relies
on the 8254 PIT. As certain x86 platforms allow that device
to be optional or turned off for power saving reasons, allow
a path where get_cpu_speed() is no longer called. Additionally,
this approach also allows the libpayload to not duplicate logic
that already exists in coreboot.

BUG=chrome-os-partner:50214
BRANCH=glados
TEST=Confirmed in payload TSC frequency is honored instead of
     using get_cpu_speed().

Change-Id: Ib8993afdfb49065d43de705d6dbbdb9174b6f2c4
Signed-off-by: Aaron Durbin <adurbin@chromium.org>
Reviewed-on: https://review.coreboot.org/13671
Tested-by: build bot (Jenkins)
Reviewed-by: Martin Roth <martinroth@google.com>
Reviewed-by: Andrey Petrov <andrey.petrov@intel.com>

payloads/libpayload/include/coreboot_tables.h

@@ -268,6 +268,14 @@ struct cb_boot_media_params {
 	uint64_t boot_media_size;
 };
 
+#define CB_TAG_TSC_INFO 0x0032
+struct cb_tsc_info {
+	uint32_t tag;
+	uint32_t size;
+
+	uint32_t freq_khz;
+};
+
 #define CB_TAG_SERIALNO		0x002a
 #define CB_MAX_SERIALNO_LENGTH	32