ath79: add support for the Belkin F9K1115 v2 (AC1750 DB Wi-Fi)

ath79: add support for the Belkin F9K1115 v2 (AC1750 DB Wi-Fi)

This device has (almost?) identical hardware to the F9J1108 v2 but uses
a different firmware magic and model number.

Specifications:

SoC: QCA9558
CPU: 720 MHz
Flash: 16 MiB NOR
RAM: 128 MiB
WiFi 2.4 GHz: QCA9558-AT4A 3x3 MIMO 802.11b/g/n
WiFi 5 GHz: QCA9880-2R4E 3x3 MIMO 802.11a/n/ac
Ethernet: 4x LAN and 1x WAN (all 1Gbit/s ports)
USB: 1 x USB 2.0 (lower), 1 x USB 3.0 (upper)

MAC addresses based on OEM firmware:

Interface   Address   Location
---------   -------   --------
lan         *:5A      sometimes in 0x6
wan         *:5B      0x0
2.4Ghz      *:5A      0x1002
5Ghz        As per mini PCIe EEPROM

Flashing instructions:

The factory.bin can be flashed via the Belkin web UI or via the uboot
HTTP upgrade page (which is by default listening on 192.168.2.1). Once
the factory.bin has been written, sysupgrade.bin will work as usual.

Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>