dt-bindings: net: add pcs-handle attribute

Document the new pcs-handle attribute to support connecting to an
external PHY. For Xilinx's AXI Ethernet, this is used when the core
operates in SGMII or 1000Base-X modes and links through the internal
PCS/PMA PHY.

Signed-off-by: Andy Chiu <andy.chiu@sifive.com>
Reviewed-by: Greentime Hu <greentime.hu@sifive.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>