ath79: add support for TP-Link Archer C60 v3

ath79: add support for TP-Link Archer C60 v3

TP-Link Archer C60 v3 is a dual-band AC1350 router,
based on Qualcomm/Atheros QCA9561 + QCA9886.

It seems to be identical to the v2 revision, except that
it lacks a WPS LED and has different GPIO for amber WAN LED.

Specification:

- 775/650/258 MHz (CPU/DDR/AHB)
- 64 MB of RAM (DDR2)
- 8 MB of FLASH (SPI NOR)
- 3T3R 2.4 GHz
- 2T2R 5 GHz
- 5x 10/100 Mbps Ethernet
- 6x LED, 2x button
- UART header on PCB

Flash instruction (WebUI):
Download *-factory.bin image and upload it via the firmwary upgrade
function of the stock firmware WebUI.

Flash instruction (TFTP):
1. Set PC to fixed IP address 192.168.0.66
2. Download *-factory.bin image and rename it to tp_recovery.bin
3. Start a tftp server with the file tp_recovery.bin 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. Wait ~30 second to complete recovery

While TFTP works for OpenWrt images, my device didn't accept the
only available official firmware "Archer C60(EU)_V3.0_190115.bin".

In contrast to earlier revisions (v2), the v3 contains the (same)
MAC address twice, once in 0x1fa08 and again in 0x1fb08.
While the partition-table on the device refers to the latter, the
firmware image contains a different partition-table for that region:

name           device            firmware
factory-boot   0x00000-0x1fb00   0x00000-0x1fa00
default-mac    0x1fb00-0x1fd00   0x1fa00-0x1fc00
pin            0x1fd00-0x1fe00   0x1fc00-0x1fd00
product-info   0x1fe00-0x1ff00   0x1fd00-0x1ff00
device-id      0x1ff00-0x20000   0x1ff00-0x20000

While the MAC address is present twice, other data like the PIN isn't,
so with the partitioning from the firmware image the PIN on the device
would actually be outside of its partition.
Consequently, the patch uses the MAC location from the device (which
is the same as for the v2).

Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>