ath79: add support for onion omega

ath79: add support for onion omega

The Onion Omega is a hardware development platform with built-in WiFi.

https://onioniot.github.io/wiki/

Specifications:
 - QCA9331 @ 400 MHz (MIPS 24Kc Big-Endian Processor)
 - 64MB of DDR2 RAM running at 400 MHz
 - 16MB of on-board flash storage
 - Support for USB 2.0
 - Support for Ethernet at 100 Mbps
 - 802.11b/g/n WiFi at 150 Mbps
 - 18 digital GPIOs
 - A single Serial UART
 - Support for SPI
 - Support for I2S

Flash instructions:
The device is running OpenWrt upon release using the ar71xx target.
Both a sysupgrade
and uploading the factory image using u-boots web-UI do work fine.

Depending on the ssh client, it might be necessary to enable outdated
KeyExchange methods e.g. in the clients ssh-config:

Host 192.168.1.1
        KexAlgorithms +diffie-hellman-group1-sha1

The stock credentials are: root onioneer

For u-boots web-UI manually configure `192.168.1.2/24` on your computer,
connect to `192.168.1.1`.

MAC addresses as verified by OEM firmware:
2G       phy0      label
LAN      eth0      label - 1

LAN is only available in combination with an optional expansion dock.

Based on vendor acked commit:
commit 5cd49bb067ca ("ar71xx: add support for Onion Omega")

Partly reverts:
commit fc553c7e4c8e ("ath79: drop unused/incomplete dts")

Signed-off-by: Jan-Niklas Burfeind <git@aiyionpri.me>