Add hypervisor register support

[lingfuyi]

Hi @andre-richter Thank you for maintaining this amazing project! This PR introduces a new interface (register HFGRTR_EL2) , This interface is currently being used in my new project [rust for pkvm] to [convert kernel nvhe code to rust].  The changes have been tested with updated test cases (and documentation). If possible, I’d appreciate it if this could be merged to benefit more developers. I’m happy to address any feedback or adjustments needed.  Thank you for your time! 🙏