tracing: Add a comment about ftrace_regs definition

To clarify what will be expected on ftrace_regs, add a comment to the
architecture independent definition of the ftrace_regs.

Signed-off-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
Acked-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>