greybus: loopback: add poll support to the iteration_count sysfs file

This adds the ability to poll on "iteration_count" in sysfs and be woken
up when it changes, saving some cycles constantly hammering on the file
waiting for it to change.

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