sle44xx: optionally use samplerate to show processing durations

The protocol is clocked, so strictly does not depend on the samplerate.
When the samplerate is available, the duration of internal processing
(memory erase and write) can get annotated. It's an optional feature.

The datasheet suggests that write and erase time are in the range of a
few milliseconds. Normalize to ms units and provide 10us resolution.