[ARM] 3579/1: AT91RM9200 Timer simplification

Patch from Andrew Victor

Use a global variable 'last_crtr' to store the time of the last timer
tick instead of the ST_RTAR register.
It's faster, frees up the ST_RTAR register for other uses, and hopefully
makes the code more understandable.  [Patch from Peter Menzebach]

Also add the SA_TIMER flag to Timer IRQ.  (It seems to be required for
the realtime preempt patch).

Signed-off-by: Andrew Victor <andrew@sanpeople.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>