dt-bindings: net: Add IEP interrupt

The IEP interrupt is used in order to support both capture events, where
an incoming external signal gets timestamped on arrival, and compare
events, where an interrupt is generated internally when the IEP counter
reaches a programmed value.

Signed-off-by: Diogo Ivo <diogo.ivo@siemens.com>
Acked-by: Conor Dooley <conor.dooley@microchip.com>
Signed-off-by: David S. Miller <davem@davemloft.net>