dt-bindings: phy: add yaml binding for rockchip,px30-dsi-dphy

This adds a yaml binding for the external dsi phy found on Rockchip
socs of the px30, rk3128 and rk3368 variants.

Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>