arm: introduce cross trigger interface helpers

OMAP4 uses cross trigger interface(CTI) to route
performance monitor irq to GIC, so introduce cti
helpers to make access for cti easily.

Acked-by: Jean Pihet <j-pihet@ti.com>
Signed-off-by: Ming Lei <ming.lei@canonical.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>