ARM: at91: remove unused IRQ function declarations

Since 3b26f39b

 (ARM: at91: make use of the new AIC driver for dt enabled
boards) the old IRQ initialisation functions aren't used anymore: remove their
declaration in generic.h.

Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Acked-by: Boris BREZILLON <boris.brezillon@free-electrons.com>