Documentation: add Broadcom SYSTEMPORT Device Tree bindings

* 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>;

};