I've lived down there where you are and worked in 117F temps for nine months out of the year, so I'm in sympathy for your chickens. At least one hour per day of direct sunlight will be plenty to absorb enough vitamin D.
As long as chickens have shade, they will be able to regulate how much heat they can handle from direct exposure by moving back and forth from sun to shade.
Here's a neat trick I learned from a desert ground squirrel, the little guys that look like chipmunks. If you dampen the ground where your chickens will be, the chickens can use that damp earth to shed the excess heat accumulating in their bodies. The laws of thermodynamics and entropy are that heat is drawn into cool.
I used to watch with admiration that tiny squirrel zipping around in 117F heat and pausing at each little bit of shady ground to prostrate itself to shed heat into that cooler spot. Here, on a 85F day, I dampen the dirt bathing area and my chickens sunbathe comfortably in the cool, damp dirt.
