ramips: add support for D-Link DIR-1935 A1

ramips: add support for D-Link DIR-1935 A1

Add support for D-Link DIR-1935 A1 based on similarities to DIR-882 A1,
DIR-867 A1 and other DIR-8xx A1 models. Existing DIR-882 A1 openwrt
"factory" firmware installs without modificaitons via the D-Link
Recovery GUI and has no known incompatibilities with the DIR-1935 A1.

Changes to be committed:
       new file:   target/linux/ramips/dts/mt7621_dlink_dir-1935-a1.dts
       modified:   target/linux/ramips/image/mt7621.mk
       modified:   target/linux/ramips/mt7621/base-files/etc/board.d/01_leds

Specifications:
* Board: Not known
* SoC: MediaTek MT7621 Family
* RAM: 128 MB (DDR3)
* Flash: 16 MB (SPI NOR)
* WiFi: MediaTek MT7615 Family (x2)
* Switch: 1 WAN, 4 LAN (Gigabit)
* Ports: 1 USB 3.0
* Buttons: Reset, WiFi Toggle, WPS
* LEDs: Power (green/orange), Internet (green/orange), WiFi 2.4G (green),
        WiFi 5G (green)

Notes:
* 160MHz 5GHz is available in LuCi but does not appear to work (i.e. no
  SSID is visible in wifi scanning apps on other devices) with either
  official DIR-882 A1 firmware or a test build for the DIR-1935 A1 based
  on the 22.03.2 branch. 80 MHz 5GHz works.

Serial port:
* Untested (potential user damage/error)
* Expected to be identical to other DIR-8xx A1 models:
	* Parameters: 57600, 8N1
	* Location: J1 header (close to the Reset, WiFi and WPS buttons)
	* Pinout: 1 - VCC
		  2 - RXD
	          3 - TXD
		  4 - GND

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>