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 <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

Documentation/devicetree/bindings/net/xilinx_axienet.txt

···
 		  1 to enable partial TX checksum offload,
 		  2 to enable full TX checksum offload
 - xlnx,rxcsum	: Same values as xlnx,txcsum but for RX checksum offload
+- xlnx,switch-x-sgmii : Boolean to indicate the Ethernet core is configured to
+		  support both 1000BaseX and SGMII modes. If set, the phy-mode
+		  should be set to match the mode selected on core reset (i.e.
+		  by the basex_or_sgmii core input line).
 - clocks	: AXI bus clock for the device. Refer to common clock bindings.
 		  Used to calculate MDIO clock divisor. If not specified, it is
 		  auto-detected from the CPU clock (but only on platforms where