Some of the MHI modems like SDX65 based ones are capable of entering the EDL
mode as per the standard triggering mechanism defined in the MHI spec v1.2. So
let's add a common mhi_pci_generic_edl_trigger() function that triggers the EDL
mode in the device when user writes to the /sys/bus/mhi/devices/.../trigger_edl
file.
Signed-off-by: Qiang Yu <quic_qianyu@xxxxxxxxxxx>