HPET: create /dev/hpetN as a way to access HPET from userland.

In some cases, TSC is broken and special applications might benefit
from memory mapping HPET and reading the registers to count time.
Most often the main HPET counter is 32-bit only[1], so this only gives
the application a 300 second window based on the default HPET
interval.
Other applications, such as Intel's DPDK, expect /dev/hpet to be
present and use it to count time as well.

Although we have an almost userland version of gettimeofday() which
uses rdtsc in userland, it's not always possible to use it, depending
on how broken the multi-socket hardware is.

Install the acpi_hpet.h so that applications can use the HPET register
definitions.

[1] I haven't found a system where HPET's main counter uses more than
32 bit.  There seems to be a discrepancy in the Intel documentation
(claiming it's a 64-bit counter) and the actual implementation (a
32-bit counter in a 64-bit memory area).

MFC after:	1 week
Relnotes:	yes