x86: Avoid timer-clock overflow

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>