ARM: EXYNOS: Add arm-pmu DT binding for exynos5250

This patch enables arm-pmu to bind device tree for exynos5250. The exynos5250
has two pmus which have combiner irq type.

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>
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>