[ARM] 4688/1: at91: speed-up irq processing

Save N instructions on every interrupt processing (where N is the
number of interrupts processed in any one IRQ exception).

Signed-off-by: Marc Pignat <marc.pignat@hevs.ch>
Acked-by: Andrew Victor <andrew@sanpeople.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>