x86/hpet: Use irq_find_matching_fwspec() to find remapping irqdomain

All possible parent domains have a select method now. Make use of it.

Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/r/20201024213535.443185-28-dwmw2@infradead.org