ARM: mmp: support DT in irq

Merge irq-pxa168 and irq-mmp2. And support device tree also.

Since CONFIG_SPARSE_IRQ is enabled in arch-mmp, base irq starts from
NR_IRQS_LEGACY.

Signed-off-by: Haojian Zhuang <haojian.zhuang@gmail.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>