Documentation/rtla: Include BPF sample collection

Add dependencies needed to build rtla with BPF sample collection support
to README, and document both ways of sample collection in the manpages.

Signed-off-by: Tomas Glozar <[e-mail]>
Acked-by: Steven Rostedt (Google) <[e-mail]>
Reviewed-by: Luis Claudio R. Goncalves <[e-mail]>
Signed-off-by: Jonathan Corbet <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]

Documentation/tools/rtla/common_timerlat_description.rst

···
 prints the timer latency at the timer *IRQ* handler and the *Thread*
 handler. It also enables the trace of the most relevant information via
 **osnoise:** tracepoints.
+
+The **rtla timerlat** tool sets the options of the *timerlat* tracer
+and collects and displays a summary of the results. By default,
+the collection is done synchronously in kernel space using a dedicated
+BPF program attached to the *timerlat* tracer. If either BPF or
+the **osnoise:timerlat_sample** tracepoint it attaches to is
+unavailable, the **rtla timerlat** tool falls back to using tracefs to
+process the data asynchronously in user space.

tools/tracing/rtla/README.txt

···
  - libtraceevent
  - libcpupower (optional, for --deepest-idle-state)
 
+For BPF sample collection support, the following extra dependencies are
+required:
+
+ - libbpf 1.0.0 or later
+ - bpftool with skeleton support
+ - clang with BPF CO-RE support
+
 It also depends on python3-docutils to compile man pages.
 
 For development, we suggest the following steps for compiling rtla: