dt-bindings: add vendor prefix and root compatible property for Rockchip boards

This adds vendor prefix and root compatible property for following boards

- Firefly, Firefly-RK3288 boards (both beta and mass production version)
- ChipSPARK, Rayeager PX2 board

PX2 SoC is fully compatible with RK3066.

Signed-off-by: FUKAUMI Naoki <naobsd@gmail.com>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>