Introduce the OLD_IRQ_AT91 Kconfig option to prepare migration to the
new AIC driver.

Select this option for all at91 SoCs and all available boards so that we
can later move DT enabled boards to the new irq driver and keep the old
implementation when legacy boards are selected.
Signed-off-by: Boris BREZILLON <boris.brezillon@free-electrons.com>
Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>