ARM: at91/aic: add irq domain and device tree support

Add an irqdomain for the AIC interrupt controller.
The device tree support is mapping the registers and
is using the irq_domain_add_legacy() to manage hwirq
translation.
The documentation is describing the meaning of the
two cells required for using this "interrupt-controller"
in a device tree node.

Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Acked-by: Jean-Christophe PLAGNIOL-VILLARD <plagnioj@jcrosoft.com>