ARM: at91: pit add DT support

Retreive registers address and IRQ from device tree entry.
Called from at91_dt_init_irq() so that timers are up-n-running
when timers initialization will occur.

Signed-off-by: Jean-Christophe PLAGNIOL-VILLARD <plagnioj@jcrosoft.com>
[nicolas.ferre@atmel.com: change error path and interrupts property handling]
Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>