dt-bindings: net: stmmac: Add the bindings documentation for XGMAC2.

Adds the documentation for XGMAC2 DT bindings.
Signed-off-by: NJose Abreu <joabreu@synopsys.com>
Cc: David S. Miller <davem@davemloft.net>
Cc: Joao Pinto <jpinto@synopsys.com>
Cc: Giuseppe Cavallaro <peppe.cavallaro@st.com>
Cc: Alexandre Torgue <alexandre.torgue@st.com>
Cc: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Cc: devicetree@vger.kernel.org
Cc: Rob Herring <robh+dt@kernel.org>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

Documentation/devicetree/bindings/net/stmmac.txt

-* STMicroelectronics 10/100/1000 Ethernet driver (GMAC)
+* STMicroelectronics 10/100/1000/2500/10000 Ethernet (GMAC/XGMAC)
 
 Required properties:
-- compatible: Should be "snps,dwmac-<ip_version>", "snps,dwmac"
+- compatible: Should be "snps,dwmac-<ip_version>", "snps,dwmac" or
+	"snps,dwxgmac-<ip_version>", "snps,dwxgmac".
 	For backwards compatibility: "st,spear600-gmac" is also supported.
 - reg: Address and length of the register set for the device
 - interrupt-parent: Should be the phandle for the interrupt controller