Some unsafe Rust comments for drivers/src/fips_test_hooks.rs

[zhalvorsen]

During an internal Google unsafe Rust review of Caliptra the reviewer had a few comments about drivers/src/fips_test_hooks.rs.

As a general comment for the file, the reviewer said the safety comments on functions appear to describe the general behavior of the function (a typical doc comment) but do not specify any soundness invariants and the unsafe code inside functions is not documented/justified.

Then specifically called out the following items:

• code let byte_0 = unsafe { &mut *mut_u8 };: This is unsound: T may have uninitialized bits, and it is illegal to dereference a pointer that exposes those.
• code let soc_ifc = unsafe { SocIfcReg::new() };: There is no safety comment explaining how SocIfcReg::new's invariants are satisfied, and those invariants are not propagated to the caller.
• code let soc_ifc = unsafe { SocIfcReg::new() };: Same as above