ramips: add support for D-Link DIR-2055 A1

ramips: add support for D-Link DIR-2055 A1

Add support for D-Link DIR-2055 A1 based on similarities to DIR-1960 A1,
as well as various DIR-8xx A1 models. Existing DIR-1960 A1 openwrt
"factory" firmware installs without modifications via the D-Link Recovery
GUI and has no known incompatibilities with the DIR-2055 A1.

Changes to be committed:
new file:   target/linux/ramips/dts/mt7621_dlink_dir-2055-a1.dts
modified:   target/linux/ramips/image/mt7621.mk
modified:   target/linux/ramips/mt7621/base-files/etc/board.d/01_leds
modified:   target/linux/ramips/mt7621/base-files/lib/upgrade/platform.sh

Specifications:

    Board: Not known
    SoC: MediaTek MT7621 Family (MT7621AT)
    RAM: 256 MB (Micron 9OK17 D9PTK, should be DDR3 MT41K128M16JT-125)
    Flash: 128 MB (Winbond W29N01HVSINA)
    WiFi: MediaTek MT7615 Family (MT7615N x2)
    Switch: 1 WAN, 4 LAN (Gigabit)
    Ports: 1 USB 3.0 (front)
    Buttons: Reset, WiFi Toggle, WPS
    LEDs: Power (white/orange), Internet (white/orange),
          WiFi 2.4G (white), WiFi 5G (white)

Notes:

    Only known difference vs. the DIR-1960 A1 is that the DIR-2055 A1
    doesn't have a USB activity LED

Serial port:

    Tested to be identical to various DIR-8xx A1 models with a similar
    enclosure/pcb design:
        Parameters: 57600, 8N1, 3.3V TTL no flow control
        Location: J1 header (close to the Reset, WiFi and WPS buttons)
        Pinout: 1 - VCC 2 - RXD 3 - TXD 4 - GND
            Did not connect VCC when using

Installation:

    D-Link Recovery GUI: power down the router, press and hold the reset
    button, then re-plug it. Keep the reset button pressed until the power
    LED starts flashing orange, manually assign a static IP address under
    the 192.168.0.xxx subnet (e.g. 192.168.0.2) and go to
    http://192.168.0.1

    Some modern browsers may have problems flashing via the Recovery GUI,
    if that occurs consider uploading the firmware through cURL:

    curl -v -i -F "firmware=@file.bin" 192.168.0.1

Signed-off-by: Keith Harrison <keithh@protonmail.com>