The call op requests that the callee pass a message to the underlying HW
or device, wait for a response, and then pass back the response error code
and message to the callee. It is useful for drivers that represent some
kind of messaging or IPC channel to a remote device.

Signed-off-by: Stephen Warren <swarren@nvidia.com>
Acked-by: Simon Glass <sjg@chromium.org>