net: fec: make PPS channel configurable

Depending on the SoC where the FEC is integrated into the PPS channel
might be routed to different timer instances. Make this configurable
from the devicetree.

When the related DT property is not present fallback to the previous
default and use channel 0.

Reviewed-by: Frank Li <Frank.Li@nxp.com>
Tested-by: Rafael Beims <rafael.beims@toradex.com>
Signed-off-by: Francesco Dolcini <francesco.dolcini@toradex.com>
Reviewed-by: Csókás, Bence <csokas.bence@prolan.hu>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>