ARM: OMAP: Simplify OMAP2 mask_irq/unmask_irq code

Modify mach-omap2/irq.c to simplify the IRQ number-to-IRQ register and
IRQ number-to-register bit calculations.

Signed-off-by: Paul Walmsley <paul@pwsan.com>
Signed-off-by: Kyungmin Park <kmpark@infradead.org>
Acked-by: Paul Mundt <lethal@linux-sh.org>
Signed-off-by: Tony Lindgren <tony@atomide.com>