arm: Remove platsupport/plat/timer.h include

This include isn't necessary for the the generic ltimer and instead
presents problems for certain platforms that do not have a platform
specific timer header file.

arm: Update handle_irq check in the generic ltimer

Now that the ARM generic ltimer is using per-CPU IRQs, this commit
updates the check in handle_irq.

trivial: Fix style

platsupport: fix compilation of generic_ltimer

It did not compile.

platsupport: add new PER_CPU irq type

This is required so irqs can specify target cpus on arm.

libplatsupport: Refactor the internal handle_irq

Now that the handle_irq functions are no longer externally visible, this
commits refactors the handle_irq functions so that they can be accessed
internally. Also add logic to invoke the user supplied callbacks after
handling the timer IRQ.

libplatsupport: Remove handle_irq from ltimers

The 'handle_irq' function is no longer necessary and shouldn't be called
externally as the IRQs will be managed by the IRQ handling functions of
the IRQ interface.

libplatsupport: Allow user callbacks for ltimer

With the IRQ changes to the ltimer, IRQs are now managed internally.
This poses a problem with the management of the time manager and other
services which rely on having known that an interrupt arrived for the
timers. This also isn't a good idea as it presents a leaky abstraction
in that the services infers events based on interrupts received.

This commit adds the ability to allow users to supply a callback
that'll be called when certain events occur for the timer, e.g. timeouts
and etc.

libplatsupport: Add IRQ register logic to ltimers

This commit adds IRQ registration logic to the ltimer driver level.
Instead of treating interrupt allocation being a OS/environment specific
thing, this commit now uses the OS/environment agnostic IRQ interface to
register these interrupts. Also fix bugs/clean-up the logic of the init
and destroy functions.

platsupport: fixes for generic_ltimer

- fix periodic timeouts
- set compare to MAX, rather than 0, when turning off comparisons or we
end up with interrupts all the time.

platsupport: add generic timer ltimer

Add an ltimer backed by the arm generic timer. For use for
debugging/testing/benchmarking when CONFIG_EXPORT_PCNT_USER is set.