Add ARM Cortex A9 Performance Monitor Unit (PMU) support.
On i.MX6 a combined interrupt on hardware line #126 is used
(i.MX6 TRM: Performance Unit interrupt).

For more details see Documentation/devicetree/bindings/arm/pmu.txt
Signed-off-by: Dirk Behme <dirk.behme@de.bosch.com>
Signed-off-by: Shawn Guo <shawn.guo@linaro.org>