x86: Avoid timer-clock overflow

When the clock speed is above about 4GHz, e.g. on modern PC hardware,
the timer overflows, resulting in a much lower frequency than expected.
Deal with this by capping the clock speed.

It would be possible to move to a 64-bit value for the clock, but that
is a pain to deal with. A better approach might be to express the clock
in MHz but that is left for later consideration.

Signed-off-by: Simon Glass <sjg@chromium.org>

drivers/timer/tsc_timer.c

@@ -442,6 +442,7 @@ static void tsc_timer_ensure_setup(bool early)
 			return;
 
 done:
+		fast_calibrate = min(fast_calibrate, 4000UL);
 		if (!gd->arch.clock_rate)
 			gd->arch.clock_rate = fast_calibrate * 1000000;
 	}