ath79: add support for TP-Link Archer D7/D7b v1

ath79: add support for TP-Link Archer D7/D7b v1

TP-Link Archer D7 v1 is a dual-band AC1750 router + modem.
The router section is based on Qualcomm/Atheros QCA9558 + QCA9880.
The "DSL" section is based on BCM6318 but it's currently not supported.

The Archer D7b seems to differ from the Archer D7 only in the
partition table.

Router section - Specification:

775/650/258 MHz (CPU/DDR/AHB)
128 MB of RAM (DDR2)
16 MB of FLASH (SPI NOR)
3T3R 2.4 GHz
3T3R 5 GHz
4x 10/100/1000 Mbps Ethernet
7x LED, 2x button
UART header on PCB

Known issues:

- Broadband LED (missing GPIO - probably driven by the BCM6318)
- Internet LED (missing GPIO - probably driven by the BCM6318)
- WIFI LED (working only for one interface at a time, while in the
  OEM firmware works for both wifi interfaces; thus, this patch does
  not set a trigger by default)
- DSL not working (eth0)

UART connection
---------------
J1 HEADER (Qualcomm CPU)
. VCC
. GND
. RX
O TX

J41 HEADER (Broadcom CPU)
. VCC
. GND
. RX
O TX

The following instructions require a connection to the J1 UART header
and are tested for the Archer D7 v1.
For the Archer D7b v1, names should be changed accordingly.

Flash instructions under U-Boot, using UART
------------------------------------------
 1. Press "tpl" to stop autobooting and obtain U-Boot CLI access.
 2. Setup ip addresses for U-Boot and your tftp server.
 3. Issue below commands:
	tftpboot 0x81000000 openwrt-ath79-generic-tplink_archer-d7-v1-squashfs-sysupgrade.bin
	erase 0x9f020000 +f90000
	cp.b 0x81000000 0x9f020000 0xf90000
	reset

Initramfs instructions under U-Boot for testing, using UART
----------------------------------------------------------
 1. Press "tpl" to stop autobooting and obtain U-Boot CLI access.
 2. Setup ip addresses for U-Boot and your tftp server.
 3. Issue below commands:
	tftpboot 0x81000000 openwrt-ath79-generic-tplink_archer-d7-v1-initramfs-kernel.bin
	bootm 0x81000000
 4. Here you can backup the original firmware and/or flash the sysupgrade openwrt if you want

Restore the original firmware
-----------------------------
 0. Backup every partition using the OpenWrt web interface
 1. Download the OEM firmware from the TP-Link website
 2. Extract the bin file in a folder (eg. Archer_D7v1_1.6.0_0.9.1_up_boot(160216)_2016-02-16_15.55.48.bin)
 3. Remove the U-Boot and the Broadcom image part from the file.
    Issue the following command:
	dd if="Archer_D7v1_1.6.0_0.9.1_up_boot(160216)_2016-02-16_15.55.48.bin" of="Archer_D7v1_1.6.0_0.9.1_up_boot(160216)_2016-02-16_15.55.48.bin.mod" skip=257 bs=512 count=31872
 4. Double check the .mod file size. It must be 16318464 bytes.
 5. Flash it using the OpenWrt web interface. Force the update if needed.
    WARNING: Remember to NOT keep settings.

 5b. (Alternative to 5.) Flash it using the U-Boot and UART connection.
     Issue below commands in the U-Boot:
	tftpboot 0x81000000 Archer_D7v1_1.6.0_0.9.1_up_boot(160216)_2016-02-16_15.55.48.bin.mod
	erase 0x9f020000 +f90000
	cp.b 0x81000000 0x9f020000 0xf90000
	reset

Signed-off-by: Davide Fioravanti <pantanastyle@gmail.com>
[cosmetic DTS changes, remove TPLINK_HWREVADD := 0, do not use two
phyXtpt at once, add missing buttons, minor commit message adjustments]
Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>