The i386 time.c code is turning into a mess. We've got multiple
functions that do the same thing, only with different hardware, all
surrounded #ifdefs and even more difficult to follow #ifndefs. George
Anzinger is introducing a new ACPIpm time source, I'm going to attempt
to add the cyclone counter as a time source, and in the future there
will be HPET to deal with. These will not go in cleanly together as
things are now.

        Inspired by suggestions from Alan, this collection of patches
tries to clean up time.c by breaking out the PIT and TSC specific parts
into their own files. Additionally the patch creates an abstract
interface to use these existing time soruces, as well as make it easier
to add future time sources.

        It introduces "struct timer_ops" which gives the time code a
clear interface to use these different time sources. It also allows for
clearer conditional compilation of these various time sources.

        This first patch (part 1 of 3) provides the infrastructure
needed via the timer_ops structure, as well as the select_timer()
function for choosing the best available timer.