ramips: fix DEVICE_VENDOR for unbranded devices

ramips: fix DEVICE_VENDOR for unbranded devices

In commit ee66fe4ea9a6 ("ramips: convert DEVICE_TITLE to new variables"),
DEVICE_VENDOR of some unbranded devices were set incorrectly:
* WR512-3GN is not a dev board from Ralink.
* "XDX-RN502J" is the whole model name and should be not split.

This patch sets their DEVICE_VENDOR to "Unbranded", and changes their DTS
model properties accordingly.

Ref: d0bf15f23595 ("ramips: add support for A5-V11 board (resubmit)")
Ref: 9085b05d9e71 ("ramips: rt305x: support for wr512-3gn-like routers")
Ref: 0e486d2fd297 ("ramips: add support for unbranded XDX-RN502J board")
Signed-off-by: Sungbo Eo <mans0n@gorani.run>