Inline resetTimer for all platforms

- for arm generic timer platforms, we remove resetTimer ->
resetGenericTimer indirection and simply include generic_timer.h
- this reduces boiler plate for platforms that share timer drivers, as
they simply include the one header
- there is far more timer code in the RT kernel, which motivates this
change

Make TIMER_CLOCK_HZ available in timer.h for ARM

The RT kernel needs this value accessible to common functions.

Add a timer.h for all ARM platforms.