dt-bindings: net: fec: add pps channel property

Add fsl,pps-channel property to select where to connect the PPS signal.
This depends on the internal SoC routing and on the board, for example
on the i.MX8 SoC it can be connected to an external pin (using channel 1)
or to internal eDMA as DMA request (channel 0).

Signed-off-by: Francesco Dolcini <francesco.dolcini@toradex.com>
Acked-by: Conor Dooley <conor.dooley@microchip.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>