ath79: add support for Sitecom WLR-8100

ath79: add support for Sitecom WLR-8100

Sitecom WLR-8100 v1 002 (marketed as X8 AC1750) is a dual band wireless
router.

Specification:

- Qualcomm Atheros SoC QCA9558
- 128 MB of RAM (DDR2)
- 16 MB of FLASH (Macronix MX25L12845EMI-10G - SPI NOR)
- 5x 10/100/1000 Mbps Ethernet
- 3T3R 2.4 GHz (QCA9558 WMAC)
- 3T3R 5.8 Ghz (QCA9880-BR4A)
- 1x USB 3.0 (Etron EJ168A)
- 1x USB 2.0
- 9x LEDs
- 2x GPIO buttons

Everything working.
Installation and restore procedure tested

Installation
1. Connect to one of LAN (yellow) ethernet ports,
2. Open router configuration interface,
3. Go to Toolbox > Firmware,
4. Browse for OpenWrt factory image with dlf extension and hit Apply,
5. Wait few minutes, after the Power LED will stop blinking, the router
	is ready for configuration.

Restore OEM FW (Linux only)
1. Download OEM FW from website (tested with WLR-8100v1002-firmware-v27.dlf)
2. Compile the FW for this router and locate the "mksenaofw" tool
	in build_dir/host/firmware-utils/bin/ inside the OpenWrt buildroot
3. Execute "mksenaofw -d WLR-8100v1002-firmware-v27.dlf -o WLR-8100v1002-firmware-v27.dlf.out" where:
	WLR-8100v1002-firmware-v27.dlf is the path to the input file
		(use the downloaded file)
	WLR-8100v1002-firmware-v27.dlf.out is the path to the output file
		(you can use the filename you want)
4. Flash the new WLR-8100v1002-firmware-v27.dlf.out file. WARNING: Do not keep settings.

Additional notes.
The original firmware has the following button configuration:
- Press for 2s the 2.4GHz button: WPS for 2.4GHz
- Press for 2s the 5GHz button: WPS for 5GHz
- Press for 15s both 2.4GHz and 5GHz buttons: Reset
I am not able to replicate this behaviour, so I used the following configuration:
- Press the 2.4GHz button: RFKILL (disable/enable every wireless interfaces)
- Press the 5GHz button: Reset

Signed-off-by: Davide Fioravanti <pantanastyle@gmail.com>