greybus: tracepoints: add tracepoints for host_device tx/rx

This patch adds new tracepoint declarations to greybus_trace.h to allow for
capture of greybus host device tx and rx events. These two tracepoints
allow an observer to see the point where the hardware interface driver
performs the relevant read or write to receive or write the data it's been
given from the higher layer greybus driver.

The following two new tracepoints are declared:
- trace_gb_host_device_send
- trace_gb_host_device_recv

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