Feature Request: Add slice support to Smc::call64 method

Hi SMCCC maintainers,

I am currently working with SMCCC's call64 implementation and would like to inquire about the possibility of adding support for passing arguments as slices (&[u64]) instead of a fixed-size array ([u64; 17]) in the call64 function.

Currently, the call64 function expects a fixed array, but in many cases, the number of arguments may vary, and using slices would provide greater flexibility, as Rust's slice type allows for dynamic-sized arguments. I understand that the parameters for SMCCC are passed via registers, and my suggestion is to allow the arguments to be passed as slices, which would internally be converted to a fixed-size array (or another mechanism that still respects the SMCCC conventions for passing arguments).

If this enhancement is deemed acceptable and aligns with the SMCCC specification, I would be happy to contribute and help implement this feature.

Thank you for considering this request. I look forward to hearing your thoughts.

Best regards,
KunWu.Chan