ramips: add support for TOTOLINK X5000R

ramips: add support for TOTOLINK X5000R

Specifications:
- SoC: MT7621AT
- RAM: 256MB
- Flash: 16MB (EN25QH128A)
- Ethernet: 5xGbE
- WiFi: MT7915 2x2 2.4G 573.5Mbps + 2x2 5G 1201Mbps

Known issue:
MT7915 DBDC variant isn't supported yet.

Flash instruction:
Upload the sysupgrade firmware to the firmware upgrade page in
vendor fw.

Other info:
MT7915 seems to have two PCIEs connected to MT7621. Card detected on
PCIE0 has an ID of 14c3:7916 and the other one on PCIE1 has 14c3:7915.

Signed-off-by: Chuanhong Guo <gch981213@gmail.com>