i386: prepare sharing the hpet code with x86_64

Add the x8664 specific bits (mapping) to share the hpet code later.

Move the reserve_platform_timer call to late init. This is necessary
for x86_64, as hpet enable() is called before memory is setup. i386
calls it in late_time_init, but it does not hurt to do it later for
both.

Pull in the x8664 hpet disable command line option as well.

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