ARM: at91: fix system timer irq issue due to sparse irq support

AT91_ID_SYS as virq is incorrect because of spare irq support which
introduces NR_IRQS_LEGACY offset.

Signed-off-by: Ludovic Desroches <ludovic.desroches@atmel.com>
Tested-by: Joachim Eastwood <joachim.eastwood@jotron.com>
Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>