spi: offload: Add offset parameter

Merge series from From git@z Thu Jan  1 00:00:00 1970
Axel Haslam <ahaslam@baylibre.com>:

Add an offset parameter that can be passed in the periodic trigger.
This is useful for example when ADC drivers implement a separate periodic
signal to trigger conversion and need offload to read the result with
some delay. While at it, add some documentation to offload periodic trigger
parameters.