lib/ukrandom: Define the driver API

lib/ukrandom: Define the driver API

Define the driver API of libukrandom. This is a generic API that
deprecates the CPU-oriented ukarch_random API.

Checkpatch-Ignore: SPACING
Signed-off-by: Michalis Pappas <michalis@unikraft.io>
Reviewed-by: Maria Pana <maria.pana4@gmail.com>
Reviewed-by: Alex Apostolescu <alexx.apostolescu@gmail.com>
Reviewed-by: Simon Kuenzer <simon@unikraft.io>
Approved-by: Simon Kuenzer <simon@unikraft.io>
GitHub-Closes: #1496