arm64: dts: rockchip: Add mt7531 dsa node to BPI-R2-Pro board

Add Device Tree node for mt7531 switch connected to gmac0.

Signed-off-by: Frank Wunderlich <frank-w@public-files.de>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>