ar71xx: add support for OCEDO Koala

ar71xx: add support for OCEDO Koala

This commit adds support for the OCEDO Koala

SOC:	Qualcomm QCA9558 (Scorpion)
RAM:    128MB
FLASH:  16MiB
WLAN1:  QCA9558 2.4 GHz 802.11bgn 3x3
WLAN2:  QCA9880 5 GHz 802.11nac 3x3
INPUT:  RESET button
LED:    Power, LAN, WiFi 2.4, WiFi 5, SYS
Serial: Header Next to Black metal shield
        Pinout is 3.3V - GND - TX - RX (Arrow Pad is 3.3V)
        The Serial setting is 115200-8-N-1.

Tested and working:
 - Ethernet
 - 2.4 GHz WiFi
 - 5 GHz WiFi
 - TFTP boot from ramdisk image
 - Installation via ramdisk image
 - OpenWRT sysupgrade
 - Buttons
 - LEDs

Installation seems to be possible only through booting an OpenWRT
ramdisk image.

Hold down the reset button while powering on the device. It will load a
ramdisk image named 'koala-uImage-initramfs-lzma.bin' from 192.168.100.8.

Note: depending on the present software, the device might also try to
pull a file called 'koala-uimage-factory'. Only the name differs, it
is still used as a ramdisk image.

Wait for the ramdisk image to boot. OpenWRT can be written to the flash
via sysupgrade or mtd.

Due to the flip-flop bootloader which we not (yet) support, you need to
set the partition the bootloader is selecting. It is possible from the
initramfs image with

 > fw_setenv bootcmd run bootcmd_1

Afterwards you can reboot the device.

Signed-off-by: David Bauer <mail@david-bauer.net>