ath79: add support for Devolo Magic 2 WIFI

ath79: add support for Devolo Magic 2 WIFI

This patch support Devolo Magic 2 WIFI, board devolo_dlan2-2400-ac.
This device is a plc wifi AC2400 router/extender with 2 Ethernet
ports, has a G.hn PLC and uses LCMP protocol from Home Grid Forum.

Hardware:
   SoC:         AR9344
   CPU:         560 MHz
   Flash:       16 MiB (W25Q128JVSIQ)
   RAM:         128 MiB DDR2
   Ethernet:    2xLAN 10/100/1000
   PLC:         88LX5152 (MaxLinear G.hn)
   PLC Flash:   W25Q32JVSSIQ
   PLC Uplink:  1Gbps MIMO
   PLC Link:    RGMII 1Gbps (WAN)
   WiFi:        Atheros AR9340 2.4GHz 802.11bgn
                Atheros AR9882-BR4A 5GHz 802.11ac
   Switch:      QCA8337, Port0:CPU, Port2:PLC, Port3:LAN1, Port4:LAN2
   Button:      3x Buttons (Reset, wifi and plc)
   LED:         3x Leds (wifi, plc white, plc red)
   GPIO Switch: 11-PLC Pairing (Active Low)
                13-PLC Enable
                21-WLAN power

MACs Details verified with the stock firmware:
   Radio1: 2.4 GHz &wmac     *:4c Art location: 0x1002
   Radio0: 5.0 GHz &pcie     *:4d Art location: 0x5006
   Ethernet        &ethernet *:4e = 2.4 GHz + 2
   PLC uplink      ---       *:4f = 2.4 GHz + 3
Label MAC address is from PLC uplink

OEM SSID: echo devolo-$(grep SerialNumber /dev/mtd1 | grep -o ...$)
OEM WiFi password: grep DlanSecurityID /dev/mtd1|tr -d -|cut -d'=' -f 2

Recommendations: Configure and link your PLC with OEM firmware
BEFORE you flash the device. PLC configuration/link should
remain in different memory and should work straight forward
after flashing.

Restrictions: PLC link detection to trigger plc red led is not
available. PLC G.hn chip is not compatible with open-plc-tools,
it uses LCMP protocol with AES-128 and requires different
software.

Notes: Pairing should be possible with gpio switch. Default
configuration will trigger wifi led with 2.4Ghz wifi traffic
and plc white led with wan traffic.

Flash instruction (TFTP):
 1. Set PC to fixed ip address 192.168.0.100
 2. Download the sysupgrade image and rename it to uploadfile
 3. Start a tftp server with the image file in its root directory
 4. Turn off the router
 5. Press and hold Reset button
 6. Turn on router with the reset button pressed and wait ~15 seconds
 7. Release the reset button and after a short time
    the firmware should be transferred from the tftp server
 8. Allow 1-2 minutes for the first boot.

Signed-off-by: Manuel Giganto <mgigantoregistros@gmail.com>