dp83640: implement programmable pin functions.

This patch adapts the dp83640 driver to allow reconfiguration of which
auxiliary function goes on which pin. The functions may be reassigned
freely with the one exception of the calibration function.

Signed-off-by: Richard Cochran <richardcochran@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>