greybus: loopback: provide interface to read all latency data-points

The current loopback code provides the minimum, maximum and average latency
values for a given test set. It would be highly useful for user-space to
have access to each one of the latency metrics in order to graph outliers.

This patch adds a simple character device interface implmenting a read()
interface that allows user-space to read out the saved latency metrics
which have been stored in a kfifo for this purpose.

A module parameter is provided to allow varying the depth of the kfifo in
order to allow a user to capture potentially large data sets. This version
sets the default depth for the kfifo at 8192 dwords.

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