A search for "baremetal raspberry pi" will return a handful of results from a few brave souls. The most common are:

• Bare Metal Programming in C
• The official "Bare Metal" forum on raspberrypi.org
• An active project from dwelch67

In addition, there's a rather dated project that got ChibiOS (my preferred RTOS) running on a Pi using the above resources. There may be a separate project for the RTOS in the future!