dt-bindings: phy: Add PHY_TYPE_DP definition

Add definition for DisplayPort phy type.

Signed-off-by: Jyri Sarha <jsarha@ti.com>
Reviewed-by: Roger Quadros <rogerq@ti.com>
Reviewed-by: Kishon Vijay Abraham I <kishon@ti.com>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>