Documentation: devicetree: add PPMU events description

Extend the documenation by events description with new 'event-data-type'
field. Add example how the event might be defined in DT.

Signed-off-by: Lukasz Luba <l.luba@partner.samsung.com>
Acked-by: Chanwoo Choi <cw00.choi@samsung.com>
Signed-off-by: Chanwoo Choi <cw00.choi@samsung.com>