Documentation: dt: net: Add binding doc for Keystone NetCP ethernet driver

The network coprocessor (NetCP) is a hardware accelerator that processes
Ethernet packets. NetCP has a gigabit Ethernet (GbE) subsystem with a ethernet
switch sub-module to send and receive packets. NetCP also includes a packet
accelerator (PA) module to perform packet classification operations such as
header matching, and packet modification operations such as checksum
generation. NetCP can also optionally include a Security Accelerator(SA)
capable of performing IPSec operations on ingress/egress packets.

Keystone SoC's also have a 10 Gigabit Ethernet Subsystem (XGbE) which
includes a 3-port Ethernet switch sub-module capable of 10Gb/s and
1Gb/s rates per Ethernet port.

NetCP Subsystem device tree layout looks something like below:

-----------------------------
  NetCP subsystem(10G or 1G)
-----------------------------
	|
	|-> NetCP Devices ->	|
	|			|-> GBE/XGBE Switch
	|			|
	|			|-> Packet Accelerator
	|			|
	|			|-> Securit...