dt-bindings: phy: Add documentation for mixel dphy

Add support for the MIXEL DPHY IP as found on NXP's i.MX8MQ SoCs.

Signed-off-by: Guido Günther <agx@sigxcpu.org>
Reviewed-by: Sam Ravnborg <sam@ravnborg.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Reviewed-by: Fabio Estevam <festevam@gmail.com>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>