i386: prepare sharing the PIT code

PIT clock events work already and the PIT handling is the same for
i386 and x86_64. x86_64 does not support PIT as a clock source, so
disable the PIT clocksource for x86_64.

Use the i386 i8253.h include file for x86_64 as well to share the
exports and the PIT constants.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Chris Wright <chrisw@sous-sol.org>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Arjan van de Ven <arjan@linux.intel.com>