ath79: add support for TP-Link Archer D50 V1

ath79: add support for TP-Link Archer D50 V1

TP-Link Archer D50 v1 is a dual-band AC1200 router + modem.
The router section is based on Qualcomm/Atheros QCA9531 + QCA9882.
The "DSL" section is based on BCM6318 but it's currently not supported.

Internally eth0 is connected to the Broadcom CPU.

Router section - Specification:

CPU: QCA9531 650/600/200 MHz (CPU/DDR/AHB)
RAM: 64 MB (DDR2)
Flash: 8 MB (SPI NOR)
Wifi 2.4GHz: QCA9531 2T2R
Wifi 5GHz: QCA9982 2T2R
4x 10/100 Mbps Ethernet
8x LED, 3x button
UART header on PCB

Known issues:
DSL not working (eth0) (WIP)

UART connection
---------------
J2 HEADER (Qualcomm CPU)
. TX
. RX
. GND
O VCC

J16 HEADER (Broadcom CPU)
O VCC
. GND
. RX
. TX

The following instructions require a connection to the J2 UART header.

Flash instruction under U-Boot, using UART
------------------------------------------
 1. Press any key 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-d50-v1-squashfs-sysupgrade.bin
	erase 0x9f020000 +$filesize
	cp.b 0x81000000 0x9f020000 $filesize
	reset

Initramfs instruction under U-Boot for testing, using UART
----------------------------------------------------------
 1. Press any key 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-d50-v1-initramfs-kernel.bin
	bootm 0x81000000

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_D50v1_0.8.0_1.3_up_boot(170223)_full_2017-02-24_09.37.45.bin)
 3. Remove the U-Boot and the Broadcom image part from the file.
    Issue the following command:
	dd if="Archer_D50v1_0.8.0_1.3_up_boot(170223)_full_2017-02-24_09.37.45.bin" of="Archer_D50v1_0.8.0_1.3_up_boot(170223)_full_2017-02-24_09.37.45.bin.mod" skip=257 bs=512 count=15616
 4. Double check the .mod file size. It must be 7995392 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_D50v1_0.8.0_1.3_up_boot(170223)_full_2017-02-24_09.37.45.bin.mod
        erase 0x9f020000 +$filesize
        cp.b 0x81000000 0x9f020000 $filesize
        reset

Signed-off-by: Davide Fioravanti <pantanastyle@gmail.com>
Signed-off-by: Christian Lamparter <chunkeey@gmail.com> [removed
default-state = "off", it's already the default, added pcie node,
fixed typo]