Cortex-R5/GNU: Document lockstep mode

This adds Section "Lockstep Mode Support."

Key additions:
- Hardware configuration requirements and boot sequence constraints
- Three practical TCM scrubbing approaches for early initialization
- Clear distinction between lockstep and SMP configurations
- Replaced pseudocode with working validation test implementations
- Added fault injection, stress testing, and error handler templates
- Lockstep vs SMP comparison and hardware requirements
- Debug/trace limitations and watchdog timer considerations
- Determinism requirements for reliable operation
- Cache/TCM behavior and interrupt timing guarantees
- Hardware reconfiguration process for mode transitions

Suitable for: ISO 26262 ASIL-D, IEC 62304 Class C deployments

Author: jserv