rtla build fails due to doc build dependency on rst2man. Make
doc build optional so rtla could be built without docs. Leave
the install dependency on doc_install alone.

Link: https://lkml.kernel.org/r/20220126001301.79096-1-skhan@linuxfoundation.orgAcked-by: Daniel Bristot de Oliveira <bristot@kernel.org>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>