ramips: add support for D-Link DIR-878 A1

ramips: add support for D-Link DIR-878 A1

Specifications:

    SoC: MT7621AT
    RAM: 128MB
    Flash: 16MB NOR SPI flash
    WiFi: MT7615N (2.4GHz) and MT7615N (5Ghz)
    LAN: 5x1000M
    Firmware layout is Uboot with extra 96 bytes in header
    Base PCB is AP-MTKH7-0002
    LEDs Power Green,Power Orange,Internet Green,Internet Orange
    LEDs "2.4G" Green & "5G" Green connected directly to wifi module
    Buttons Reset,WPS,WIFI

Flashing instructions:

Upload image via emergency recovery mode
Push and hold reset button (on the back of the device) until power led
starts flashing (about 10 secs or so) while powering the device on.
Give it ~30 seconds, to boot the recovery mode GUI
Connect your client computer to LAN1 of the device
Set your client IP address manually to 192.168.0.2 / 255.255.255.0.
Call the recovery page for the device at http://192.168.0.1
Use the provided emergency web GUI to upload and flash a new firmware to
the device. Some browsers/OS combinations are known not to work, so if
you don't see the percentage complete displayed and moving within a few
seconds, restart the procedure from scratch and try anoher one,
or try the command line way.

Alternative method using command line on Linux:
curl -v -i -F "firmware=@openwrt-xxxx-squashfs-factory.bin" 192.168.0.1

Signed-off-by: Mathieu Martin-Borret <mathieu.mb@protonmail.com>
[use of generic uimage-padhdr in image generation code]
Signed-off-by: Petr Štetiar <ynezz@true.cz>