ramips: mt7621: add support for D-Link DIR-X1860 B1 / DIR-X1550 A1

ramips: mt7621: add support for D-Link DIR-X1860 B1 / DIR-X1550 A1

Both devices seem to be completely identical and D-Link doesnt even
mention the DIR-X1550 A1 in the GPL source. Furthermore the supported
devices header also just contains DIR-X1860 B1. The cherry on top is the
FCC filing, which features the manual for DIR-X1550 A1 but the label
info for DIR-X1860 B1. I guess someone at D-Link was just as confused as
me.

Hardware
--------
SOC:    MediaTek MT7621AT
FLASH:	128MB (Spansion S34ML01G200TF100)
RAM:  	256MB (Winbond W632GU6NB-12)
WIFI:   MediaTek MT7915DAN + MT7975DN DBDC 2x2 802.11ax
ETH:	1x WAN, 3x LAN
LED:	6 (4 GPIO controllable, 2 WIFI hardwired)
BTN:	WPS, Reset
UART:	115200 8N1 (Pinout silkscreened) - ignore VCC

MAC addresses
-------------
LAN	Label MAC (stored in config2 partition as ASCII (entry
	factory_mac=xx:xx:xx:xx:xx:xx))
WAN	LAN + 3
2.4G	LAN + 1
5G	LAN + 2

Installation
------------
Vendor UI
---------
1. Browse to http://192.168.0.1 and login.
2. Navigate to "Management" -> "Upgrade".
3. Press the "Select File" button and upload
   openwrt-ramips-mt7621-dlink_dir-x1860-b1-squashfs-factory.bin
4. Confirm the security questions, wait for a reboot and enjoy OpenWrt.

Recovery UI
-----------
1. Set your IP address to 192.168.0.101, subnet 255.255.255.0.
2. Power on the device while holding reset.
3. Release reset once the status led starts to blink orange.
4. Open a chrome- or firefox based browser and browse to
   https://192.168.0.1
5. Upload openwrt-ramips-mt7621-dlink_dir-x1860-b1-squashfs-recovery.bin
   wait for a reboot and enjoy OpenWrt.

Back to stock
-------------
1. Set your IP address to 192.168.0.101, subnet 255.255.255.0.
2. Power on the device while holding reset.
3. Release reset once the status led starts to blink orange.
4. Open a chrome- or firefox based browser and browse to
   https://192.168.0.1
5. Upload a decrypted vendor image, wait for a reboot and regret your
   decision.

Decrypt vendor image
--------------------
1. Download dlink-sge-image.c and dlink-sge-image.h from the
   firmware-utils openwrt repository.
2. Compile a binary from the downloaded file
   e.g. gcc dlink-sge-image.c -lcrypto -o dlink-sge-image
3. Run
   ./dlink-sge-image DIR-X1860-B1 <vendor_image> <decrypted_image> -d

Signed-off-by: Christoph Krapp <achterin@gmail.com>
Link: https://github.com/openwrt/openwrt/pull/20410
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>