csky: add CONFIG_CSKY_NR_IRQS to define NR_IRQS.

Some platform need to use gpio irq, and NR_IRQS should be defined in a
flexiable way.

Change-Id: Ie87b944a64bd169cfb640e8bb829926f3fe989da
Signed-off-by: Guo Ren <ren_guo@c-sky.com>