[IA64] ITC: Reduce rating for ITC clock if ITCs are drifty

Make sure to reduce the rating of the ITC clock if ITCs are drifty. If they
are drifting then we have not synchronized the ITC values, nor are we doing
the jitter compensation (useless since drift may increase the differentials
arbitrarily).

Without this patch it is possible that the ITC clock becomes selected as
the system clock on systems with drifty ITCs which will result in
nanosleep hanging.

One can still select the itc clock manually on such systems via

clocksource=itc

(Produces nice hangs on SGI Altix.)

Signed-off-by: Christoph Lameter <clameter@sgi.com>
Signed-off-by: Tony Luck <tony.luck@intel.com>