greybus: loopback: Move loopback operation definitions

In order to have one point of type definition for gbsim move the loopback
operation definitions to greybus_protocols.h.

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