spi: offload: Add offset parameter

spi: offload: Add offset parameter

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.

Reviewed-by: David Lechner <dlechner@baylibre.com>
Signed-off-by: Axel Haslam <ahaslam@baylibre.com>
Signed-off-by: Marcelo Schmitt <marcelo.schmitt@analog.com>
Link: https://patch.msgid.link/cd315e95c0bd8523f00e91c400abcd6a418e5924.1759760519.git.marcelo.schmitt@analog.com
Signed-off-by: Mark Brown <broonie@kernel.org>