ARM: EXYNOS: Add arm-pmu DT binding for exynos421x

This patch adds a arm-pmu node to bind device tree for exynos4210.
The exynos4210 and 4212 have two cpus which includes a pmu. In contrast, the
exynos4412 has 4 cpus and pmus. We need to define two more pmus for this type
board. However, supporting arm-pmu for the exynos4412 will handle it later
because there is no dts support for 4412 based board.

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>