ARM: at91: fix at91_extern_irq usage for non-dt boards

Since 4b68520dc0ec96153bc0d87bca5ffba508edfcf
ARM: at91: add AIC5 support

we allocate the at91_extern_irq.

This patch makes it static and stores the non-dt extern irq in the soc
structure. It is then possible to use a at91_get_extern_irq() function
to get the value for outside of the irq driver. It is useful for passing
its value to at91_aic_init().

Signed-off-by: Jean-Christophe PLAGNIOL-VILLARD <plagnioj@jcrosoft.com>
Acked-by: Ludovic Desroches <ludovic.desroches@atmel.com>
[nicolas.ferre@atmel.com: rework commit message]
Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>