arm64: dts: rockchip: add stdout path on Qnap-TS433

As most Rockchip boards do, the TS433 also uses uart2 for its serial
output. Set the correct chosen entry for it.

Tested-by: Uwe Kleine-König <ukleinek@debian.org>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Link: https://lore.kernel.org/r/20240723195538.1133436-6-heiko@sntech.de