arm: prima2: drop nr_irqs in mach as we moved to linear irqdomain

we don't need nr_irqs in machine any more after we move to
linear irqdomain for sirfsoc irqchip, so drop them.

Signed-off-by: Barry Song <Baohua.Song@csr.com>
Signed-off-by: Olof Johansson <olof@lixom.net>