ramips: add support for D-Link DIR-878 R1

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

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:
Update to the last D-Link firmware through web-ui before OpenWRT
installation then follow the instructions to patch your device using
D-Link FailsafeUI.

- 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 192.168.0.1 (e.g. 192.168.0.2)
and go to http://192.168.0.1


Flash with the factory image.

Signed-off-by: Igor Nazarov <tigron.dev@gmail.com>