ramips: add support for JCG JHR-AC876M

ramips: add support for JCG JHR-AC876M

JCG JHR-AC876M is an AC2600M router

Hardware specs:
  SoC: MT7621AT
  2.4GHz: MT7615N 4x4 @ PCIe0
  5GHz: MT7615N 4x4 @ PCIe1
  Flash: Winbond W25Q128JVSQ 16MiB
  RAM: Nanya NT5CB128M16 256MiB
  USB 2.0 and 3.0 ports
  6 LEDs, 3 of which are connected to SoC GPIO
  Reset and WPS buttons

Flash instructions:
Stock to OpenWrt:
  Upload factory.bin in stock firmware's upgrade page,
  do not preserve settings

OpenWrt to stock:
  Push and hold the reset button for 5s while power cycling to
  enter recovery mode;
  Visit 192.168.1.1 and upload stock firmware

MAC addresses map:
  0x0004  *:1c  wlan2g/wan/label
  0x8004  *:20  wlan5g
  0xe000  *:1b  lan
  0xe006  *:1a  not used in stock fw

Signed-off-by: DENG Qingfang <dengqf6@mail2.sysu.edu.cn>