dt-bindings: net: xilinx_axienet: add xlnx,switch-x-sgmii attribute

Document the new xlnx,switch-x-sgmii attribute which is used to indicate
that the Ethernet core supports dynamic switching between 1000BaseX and
SGMII.

Signed-off-by: Robert Hancock <robert.hancock@calian.com>
Signed-off-by: David S. Miller <davem@davemloft.net>