Documentation: add Broadcom SYSTEMPORT Device Tree bindings

Add the Device Tree bindings documentation for the Broadcom SYSTEMPORT
Ethernet MAC controller hardware.
Signed-off-by: NFlorian Fainelli <f.fainelli@gmail.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

Documentation/devicetree/bindings/net/broadcom-systemport.txt

+* Broadcom BCM7xxx Ethernet Systemport Controller (SYSTEMPORT)
+
+Required properties:
+- compatible: should be one of "brcm,systemport-v1.00" or "brcm,systemport"
+- reg: address and length of the register set for the device.
+- interrupts: interrupts for the device, first cell must be for the the rx
+  interrupts, and the second cell should be for the transmit queues
+- local-mac-address: Ethernet MAC address (48 bits) of this adapter
+- phy-mode: Should be a string describing the PHY interface to the
+  Ethernet switch/PHY, see Documentation/devicetree/bindings/net/ethernet.txt
+- fixed-link: see Documentation/devicetree/bindings/net/fsl-tsec-phy.txt for
+  the property specific details
+
+Optional properties:
+- systemport,num-tier2-arb: number of tier 2 arbiters, an integer
+- systemport,num-tier1-arb: number of tier 1 arbiters, an integer
+- systemport,num-txq: number of HW transmit queues, an integer
+- systemport,num-rxq: number of HW receive queues, an integer
+
+Example:
+ethernet@f04a0000 {
+	compatible = "brcm,systemport-v1.00";
+	reg = <0xf04a0000 0x4650>;
+	local-mac-address = [ 00 11 22 33 44 55 ];
+	fixed-link = <0 1 1000 0 0>;
+	phy-mode = "gmii";
+	interrupts = <0x0 0x16 0x0>,
+		<0x0 0x17 0x0>;
+};