Enable SOF events on nRF52840 to fix USB latency
Enable SOF events on nRF52840 to fix USB latency

CDC device-to-host transmission is started on the SOF interrupt.
Without this interrupt CDC device-to-host transmission only start
if the is an unrelated USB interrupt to start transmission. This adds
a delay when sending data.

This patch enables the SOF interrupt so CDC device-to-host
transmissions are deterministically started on the next USB frame.

google/DAPLink-port,google/DAPLink-port,google/DAPLink-port,google/DAPLink-port