ARM Cortex-R for motion control application
Cortex-R series are used in pretty much industrial-specific applications such as DSP core fore automotive, digital cam, or SSD controllers. Because of the nature of industry world there is not much resource available other than ARM's TRM on internet yet. About thr development board, I found TI Herculus family has the cheepest development boards called BoosterPack ($20-$50).
I believe Cortex M4 and Cortex M7 can do hard-realtime too though.