x86: add support for Mellanox Spectrum SN2000 Switches

x86: add support for Mellanox Spectrum SN2000 Switches

The Mellanox Spectrum SN2000 Series Switches are Managed Ethernet
Switches with a maximum speed of 100Gb/s and up to 56 ports.

Tested on a Mellanox Spectrum SN2100 with the following specs:
 - CPU: Intel ATOM x86 dual-core 2.4GHz
 - RAM: 8GB
 - Disk: 16GB SSD
 - Ports: 16x QSFP28 100GbE, 1x 100M Mgmt Port, 1x RJ45 Serial Port
 - USB: 1x mini 2.0
 - Button: 1x (reset)
 - LEDs: 6x

Installation:
 - Create a bootable USB device (either by flashing this image
   onto it or another Linux distribution)
 - Unzip the generic OpenWrt x64 image
   and copy it onto the USB device
 - Plug the USB device into the Mellanox Switch and boot from it
 - Flash the image (e.g., with dd) onto the internal SSD
   of the switch (should be /dev/sda)

To enter the BIOS, reboot the switch and press CTRL+B while you see
the BIOS information text (American Megatrends …). The default password
to enter the BIOS is admin. To boot from the USB device, switch to the
Boot index tab and set your USB device at the top of the boot order
(the internal SSD should be currently there). Don't forget to set
the SSD back at the top after you have flashed the image.

Signed-off-by: Til Kaiser <til.kaiser@gmx.de>
[unify with generic x64 image]
Signed-off-by: David Bauer <mail@david-bauer.net>