userspace: update k_object API to support immutable objects

The k_object API associates mutable state structures with known kernel
objects to support userspace.  The kernel objects themselves are not
modified by the API, and in some cases (e.g. device structures) may be
const-qualified.  Update the API so that pointers to these const
kernel objects can be passed without casting away the const qualifier.

Fixes #27399

Signed-off-by: Peter Bigot <peter.bigot@nordicsemi.no>