irqchip: exynos-combiner: Add set_irq_affinity function for combiner_irq

This patch adds set_irq_affinity function for combiner_irq. We need this
function to enable a arm-pmu because the pmu of exynos has combined type
irqs.

Reviewed-by: Thomas Abraham <thomas.abraham@linaro.org>
Signed-off-by: Chanho Park <chanho61.park@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
[kgene.kim@samsung.com: changes moved into drivers/irqchip/]
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>