arm: omap: irq: introduce omap_nr_pending

that variable will tell us how many INTC_PENDING_IRQn
registers we have. It'll be used on a following patch
to cleanup omap_intc_handle_irq() a bit.

Signed-off-by: Felipe Balbi <balbi@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>