RTX5: Add priority
RTX5: Add priority

Implement priority property in RTX5TargetThread, and add to description,
matching FreeRTOS implementation.

mesheven/pyOCD,mbedmicro/pyOCD,mbedmicro/pyOCD,flit/pyOCD,flit/pyOCD,mesheven/pyOCD,pyocd/pyOCD,pyocd/pyOCD,mesheven/pyOCD,mbedmicro/pyOCD