dt-bindings: net: Expand pcs-handle to an array

This allows multiple phandles to be specified for pcs-handle, such as
when multiple PCSs are present for a single MAC. To differentiate
between them, also add a pcs-handle-names property.

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>