Do not use the ia64 clocksource on non-ia64 architectures

The HPET clocksource in drivers/char/hpet.c was written as generic code
for ia64, but it is not yet ready to replace the native HPET clocksource
implementations that the i386/x86-64 architectures use.

On x86[-64], trying to register this clocksource results in potentially
multiple hpet-based clocksources being registered, and if the ia64 one
is chosen on x86_64 some users have experienced hangs.

Eventually all three architectures may end up using the same code, but
that is not the case right now.

Cc: John Stultz <johnstul@us.ibm.com>
Cc: Tony Luck <tony.luck@intel.com>
Cc: Paolo Ornati <ornati@fastwebnet.it>
Cc: Bob Picco <bob.picco@hp.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>