net: ti: icssg-prueth: Add ICSSG ethernet driver

This is the Ethernet driver for TI AM654 Silicon rev. 2
with the ICSSG PRU Sub-system running dual-EMAC firmware.

The Programmable Real-time Unit and Industrial Communication Subsystem
Gigabit (PRU_ICSSG) is a low-latency microcontroller subsystem in the TI
SoCs. This subsystem is provided for the use cases like implementation of
custom peripheral interfaces, offloading of tasks from the other
processor cores of the SoC, etc.

Every ICSSG core has two Programmable Real-Time Unit(PRUs),
two auxiliary Real-Time Transfer Unit (RT_PRUs), and
two Transmit Real-Time Transfer Units (TX_PRUs). Each one of these runs
its own firmware. Every ICSSG core has two MII ports connect to these
PRUs and also a MDIO port.

The cores can run different firmwares to support different protocols and
features like switch-dev, timestamping, etc.

It uses System DMA to transfer and receive packets and
shared memory register emulation between the firmware and
driver for control and configuration.

This patch adds support for basic EMAC functionality with 1Gbps
and 100Mbps link speed. 10M and half duplex mode are not supported
currently as they require IEP, the support for which will be added later.
Support for switch-dev, timestamp, etc. will be added later
by subsequent patch series.

Signed-off-by: Roger Quadros <rogerq@ti.com>
Signed-off-by: Vignesh Raghavendra <vigneshr@ti.com>
Signed-off-by: Grygorii Strashko <grygorii.strashko@ti.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: MD Danish Anwar <danishanwar@ti.com>
Signed-off-by: David S. Miller <davem@davemloft.net>