x86: configure HPET_EMULATE_RTC automatically

It needs to be automatic. The HPET legacy interrupt mode disconnects
the RTC interrupt and connects the interrupt of the second HPET channel.

[ tglx: arch/x86 adaptation and comment fixup]

Signed-off-by: Stefan Richter <stefanr@s5r6.in-berlin.de>
Signed-off-by: Andi Kleen <ak@suse.de>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>