GENERIC_TIME and GENERIC_CLOCKEVENTS support for the at91rm9200.

 - Oneshot mode (used for NO_HZ and high res timers) uses the
   alarm to emulate a real oneshot timer; the trickiest bit is
   how to avoid some lowlevel races.  Thanks to Remy Bohmer for
   various fixes to this code.

 - Tighten up periodic mode support using the PIT.

 - Streamline reads of the 32KHz counter.  Thanks to Marc Pignat
   for some testing results: the CRTR register has *very* odd
   behavior.  The reread appears to work around stranger glitches
   than just getting an old clock value (which would quickly
   self-correct).

 - Remove the rounding-up of tick_usec to 10.009 msec (32KiHz/100),
   since that no longer acts correct (time increases too fast).

Note that the at91sam9 and at91x40 chips need other solutions,
since they don't have the same system timer module.
Signed-off-by: NDavid Brownell <dbrownell@users.sourceforge.net>
Acked-by: NBill Gatliff <bgat@billgatliff.com>
Acked-by: NRemy Bohmer <linux@bohmer.net>
Signed-off-by: NRussell King <rmk+kernel@arm.linux.org.uk>