dp83640: enable six external events and one periodic output

This patch enables six external event channels and one periodic output.
One GPIO is reserved for synchronizing multiple PHYs. The assignment
of GPIO functions can be changed via a module parameter.

The code supports multiple simultaneous events by inducing a PTP clock
event for every channel marked in the PHY's extended status word.

Signed-off-by: Richard Cochran <richard.cochran@omicron.at>
Signed-off-by: David S. Miller <davem@davemloft.net>