irqchip: add irq-omap-intc.h header

OMAP INTC irqchip driver will be moved under
drivers/irqchip/ soon but we still have a dependency
with mach-omap2 when it comes to idle functions.

In order to make it easy to share those function
prototypes with OMAP PM code, we introduce this new
header.

To avoid modifying several board-files and some of
the PM-related code, we just include the new header
from common.h which was already included by all
users of IRQ-related PM code.

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