Add sysfs entry to allow users of MHI bus force device to enter EDL.
Considering that the way to enter EDL mode varies from device to device and
some devices even do not support EDL. Hence, add a callback edl_trigger in
mhi controller as part of the sysfs entry to be invoked and MHI core will
only create EDL sysfs entry for mhi controller that provides edl_trigger
callback. All of the process a specific device required to enter EDL mode
can be placed in this callback.
Signed-off-by: Qiang Yu <quic_qianyu@xxxxxxxxxxx>