dt-bindings: net: Add Lynx PCS binding

This binding is fairly bare-bones for now, since the Lynx driver doesn't
parse any properties (or match based on the compatible). We just need it
in order to prevent the PCS nodes from having phy devices attached to
them. This is not really a problem, but it is a bit inefficient.

This binding is really for three separate PCSs (SGMII, QSGMII, and XFI).
However, the driver treats all of them the same. This works because the
SGMII and XFI devices typically use the same address, and the SerDes
driver (or RCW) muxes between them. The QSGMII PCSs have the same
register layout as the SGMII PCSs. To do things properly, we'd probably
do something like

	ethernet-pcs@0 {
		#pcs-cells = <1>;
		compatible = "fsl,lynx-pcs";
		reg = <0>, <1>, <2>, <3>;
	};

but that would add complexity, and we can describe the hardware just
fine using separate PCSs for now.

Signed-off-by: Sean Anderson <sean.anderson@seco.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>