ARM: dts: rockchip: add Netxeon R89 board

This board is used in some TV-boxes like for example the Beelink R89 or
Tronsmart R28.

The board itself follows the reference design for the most part. But
there are no schematics available it seems, so some things should be
taken with a grain of salt.

Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Reviewed-by: Romain Perier <romain.perier@gmail.com>