net: arc_emac: add phy reset is optional for device tree

This patch adds the following property for arc_emac.

1) phy-reset-gpios:
The phy-reset-gpio is an optional property for arc emac device tree boot.
Change the binding document to match the driver code.

2) phy-reset-duration:
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.

Anyway, we can add the above property for arc emac.

Signed-off-by: Caesar Wang <wxt@rock-chips.com>
Cc: Rob Herring <robh+dt@kernel.org>
Cc: devicetree@vger.kernel.org
Cc: netdev@vger.kernel.org
Cc: "David S. Miller" <davem@davemloft.net>
Cc: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Cc; Alexander Kochetkov <al.kochet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>