[IA64] Convert to generic timekeeping/clocksource

This is a merge of Peter Keilty's initial patch (which was
revived by Bob Picco) for this with Hidetoshi Seto's fixes
and scaling improvements.

Acked-by: Bob Picco <bob.picco@hp.com>
Signed-off-by: Tony Luck <tony.luck@intel.com>