ramips: add support for D-Link DIR-882 R1

ramips: add support for D-Link DIR-882 R1

Specifications:
- SoC: MediaTek MT7621AT
- RAM: 128 MB (DDR3)
- Flash: 16 MB (SPI NOR)
- WiFi: MediaTek MT7615N (x2)
- Switch: 1 WAN, 4 LAN (Gigabit)
- Ports: 1 USB 2.0, 1 USB 3.0
- Buttons: Reset, WiFi Toggle, WPS
- LEDs: Power, Internet, WiFi 2.4G WiFi 5G, USB 2.0, USB 3.0

The R1 revision is identical to the A1 revision except
- No Config2 Parition, therefore
- factory partition resized to 64k from 128K
- Firmware partition offset is 0x50000 not 0x60000
- Firmware partitions size increased by 64K
- Firmware partition type is "denx,uimage", not "sge,uimage"
- Padding of image creation "uimage-padhdr 96" removed

Installation:
- Older firmware versions: put the factory image on a USB stick, turn on
the telnet console, and flash using the following cmd
"fw_updater Linux /mnt/usb_X_X/firmware.bin"

- D-Link FailsafeUI:
Power down the router, press and hold the reset button, then
re-plug it. Keep the reset button pressed until the internet LED stops
flashing, then jack into any lan port and manually assign a static IP
address in 192.168.0.0/24 other than 192.168.0.0 (e.g. 192.168.0.2)
and go to http://192.168.0.1
Flash with the factory image.

Signed-off-by: Andrew Pikler <andrew.pikler@gmail.com>