[ARM] 4989/1: [AT91] SAM9 ClockSource / ClockEvents

Update AT91SAM9/CAP9 PIT driver to use generic time and clockevent
infrastructure:

 - Clocksource gives sub-microsecond timestamp precision, assuming
 memory is clocked at over 16 MHz.  It's less than a 32 bit counter,
 unless it's is also generating IRQs.

 - Clockevent device supports periodic mode only; no oneshot
 support from this hardware.  No IRQs generated unless it's the
 active clocksource.

Later, another timer (probably from a TC module) can provide a oneshot
clockevent device to get NO_HZ and High-Res-Timer behavior.

This also updates the timekeeping to use the actual master clock rate
on the system, instead of compile-time <asm/arch/timex.h> constants
matching what Atmel's EK boards use.  (Product boards may well differ!)

Plus cleanup:  rename "*_timer*" symbols to "*_pit*" (there are other
timers, but only one PIT); shorter lines; remove needless CPP stuff;
make several symbols static; etc.

Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Andrew Victor <linux@maxim.org.za>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>