ARM: sirf: fix prima2 interrupt lookup

We must not read the interrupts property manually but instead
use irq_of_parse_and_map() to guarantee that we get the correct
interrupt number once we stop using the legacy IRQ domain.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Tested-by: Barry Song <Baohua.Song@csr.com>