dt-bindings: display: rockchip-dsi: add rk3568 compatible

The rk3568 uses the same dw-mipi-dsi controller as previous Rockchip
SOCs, so add a compatible string for it.

Signed-off-by: Chris Morgan <macromorgan@hotmail.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20220906174823.28561-2-macroalpha82@gmail.com