cpu/x86/tsc: Flip and rename TSC_CONSTANT_RATE to UNKNOWN_TSC_RATE

The x86 timers are a bit of a mess. Cases where different stages use
different counters and timestamps use different counters from udelays.

The original intention was to only flip TSC_CONSTANT_RATE Kconfig
to NOT_CONSTANT_TSC_RATE. The name would be incorrect though, those
counters do run with a constant rate but we just lack tsc_freq_mhz()
implementation for three platforms.

Note that for boards with UNKNOWN_TSC_RATE=y, each stage will have a
slow run of calibrate_tsc_with_pit(). This is easy enough to fix with
followup implementation of tsc_freq_mhz() for the platforms.

Implementations with LAPIC_MONOTONIC_TIMER typically will not have
tsc_freq_mhz() implemented and default to UNKNOWN_TSC_RATE. However,
as they don't use TSC for udelay() the slow calibrate_tsc_with_pit()
is avoided.

Because x86/tsc_delay.tsc was using two different guards and nb/via/vx900
claimed UDELAY_TSC, but pulled UDELAY_IO implementation, we also switch
that romstage to use UDELAY_TSC.

Change-Id: I1690cb80295d6b006b75ed69edea28899b674b68
Signed-off-by: Kyösti Mälkki <kyosti.malkki@gmail.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/33928
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Aaron Durbin <adurbin@chromium.org>

src/arch/x86/cpu.c

@@ -315,7 +315,7 @@ void lb_arch_add_records(struct lb_header *header)
 	struct lb_tsc_info *tsc_info;
 
 	/* Don't advertise a TSC rate unless it's constant. */
-	if (!CONFIG(TSC_CONSTANT_RATE))
+	if (!tsc_constant_rate())
 		return;
 
 	freq_khz = tsc_freq_mhz() * 1000;

src/arch/x86/timestamp.c

@@ -22,7 +22,7 @@ uint64_t timestamp_get(void)
 int timestamp_tick_freq_mhz(void)
 {
 	/* Chipsets that have a constant TSC provide this value correctly. */
-	if (CONFIG(TSC_CONSTANT_RATE))
+	if (tsc_constant_rate())
 		return tsc_freq_mhz();
 
 	/* Filling tick_freq_mhz = 0 in timestamps-table will trigger