include: dt-bindings: add Performance Monitoring Unit for Exynos

This patch add support of a new feature which can be used in DT:
Performance Monitoring Unit with defined event data type.
In this patch the event data types are defined for Exynos PPMU.
The patch also updates the MAINTAINERS file accordingly and
adds the header file to devfreq event subsystem.

Acked-by: Chanwoo Choi <cw00.choi@samsung.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Lukasz Luba <l.luba@partner.samsung.com>
Signed-off-by: Chanwoo Choi <cw00.choi@samsung.com>