ARM: at91: make use of the new AIC driver for dt enabled boards

Remove selection of OLD_IRQ_AT91 when selecting dt boards.
Select ATMEL_AIC_IRQ for sama5 SoCs (a kernel compiled for this SoC will
always use ATMEL_AIC_IRQ driver).
Select ATMEL_AIC_IRQ for at91rm9200 and at91sam9 SoCs only if OLD_IRQ_AT91
is not selected (which means we are compiling a pure DT kernel, without
any legacy board support).

Remove specific irq init code in all dt board files: this init procedure
is automatically handled in of_irq_init which is called by the arm irq core
code and is in charge of calling the appropriate aic init functions.

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