ARM: at91: sparse irq support

Enable sparse irq support for multisoc image. It involves to add the
NR_IRQS_LEGACY offset to static SoC irq number definitions since NR_IRQS_LEGACY
irq descs are allocated before AIC requests irq descs allocation.
Move NR_AIC_IRQS macro to a more appropiate place with the purpose to
remove mach/irqs.h later.

Signed-off-by: Ludovic Desroches <ludovic.desroches@atmel.com>
Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>