Usually, light poles are supported by small, drilled concrete piers of sufficient diameter to allow proper bolting of the pole to the pier.
The design involves considering the lateral forces on the pier and the desired amount of lateral deflection that can be tolerated. A geotechnical engineer can help with this in terms of allowable stresses on the soil and determining an appropriate soil modulus to measure its stiffness.
Your local municipality may also have some set standards for light poles, traffic poles, etc. that you could utilize.