net: dsa: add Arrow SpeedChips XRS700x driver

Add a driver with initial support for the Arrow SpeedChips XRS7000
series of gigabit Ethernet switch chips which are typically used in
critical networking applications.

The switches have up to three RGMII ports and one RMII port.
Management to the switches can be performed over i2c or mdio.

Support for advanced features such as PTP and
HSR/PRP (IEC 62439-3 Clause 5 & 4) is not included in this patch and
may be added at a later date.

Signed-off-by: George McCollister <george.mccollister@gmail.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Vladimir Oltean <olteanv@gmail.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>