[ARM] 4545/1: ns9xxx: simplify irq ack'ing

Now the drivers are responsible to clear the irq in the respective
device, which seems to be the normal thing to do.

So the ack'ing of the timer irq moved to time.c.

Signed-off-by: Uwe Kleine-König <ukleinek@informatik.uni-freiburg.de>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>