greybus: tracepoints: add tracepoint definitions

This patch adds greybus_trace.h with the following trace definitions

- trace_gb_message_send
- trace_gb_message_recv_request
- trace_gb_message_recv_response
- trace_gb_message_cancel_incoming
- trace_gb_message_cancel_outgoing

Signed-off-by: Bryan O'Donoghue <bryan.odonoghue@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>