rtla: Documentation: Mention --deepest-idle-state

Add --deepest-idle-state to manpage and mention libcpupower dependency
in README.txt.

Link: https://lore.kernel.org/20241017140914.3200454-7-tglozar@redhat.com


Signed-off-by: Tomas Glozar <tglozar@redhat.com>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>