Microchip PolarFire SoC Peripheral Support

Implement platform services required by the condition-monitoring demo.

Goals:

- MMUART serial console support (16550A)
- CLINT 64-bit machine timer integration (10ms tick)
- PLIC interrupt controller integration
- virtual sensor abstraction
- hardware abstraction cleanup

DoD:

- [ ] UART serial validation
- [ ] Timer tick & interrupt validation
- [ ] Peripheral documentation