x86/hpet: Move MSI support into hpet.c

This isn't really dependent on PCI MSI; it's just generic MSI which is now
supported by the generic x86_vector_domain. Move the HPET MSI support back
into hpet.c with the rest of the HPET support.

Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/r/20201024213535.443185-11-dwmw2@infradead.org