net: fec: add phy-reset-duration for device tree probe

Different boards may require different phy reset duration.  Add property
phy-reset-duration for device tree probe, so that the boards that need
a longer reset duration can specify it in their device tree.

Signed-off-by: Shawn Guo <shawn.guo@linaro.org>
Signed-off-by: David S. Miller <davem@davemloft.net>