Change in flashrom[master]: [RFC] spi_master: Add shutdown function in spi_master struct

[Anastasia Klimchuk (Code Review)]

Attention is currently required from: Nico Huber, Edward O'Callaghan, Angel Pons.
Anastasia Klimchuk has posted comments on this change. ( https://review.coreboot.org/c/flashrom/+/55932 )

Change subject: [RFC] spi_master: Add shutdown function in spi_master struct
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Patch Set 1:

(1 comment)

Patchset:

PS1: 
I was thinking about adding a shutdown function into one of the structs for a while (I mean into spi_master struct or programmer_entry). I wasn't sure which one of the structs is the right place for the shutdown function, and even more, why there are two structs describing the same thing? Especially now, when both structs are in the same file. Both in the same file, however they are separate and cannot share information between each other.

Programmer_entry has init function, so I was thinking of adding a shutdown there as well. But then I cannot access it from register_spi_master. Alternatively, I thought to add `register_shutdown(programmer->shutdown, data)` after flashrom.c#159, but data is not available on that level.

Data is needed for shutdown, but also data is something that is re-created for every new run. With that, maybe adding shutdown into spi_master is ok? Data is there already.

Another thing I don't know, why data is added into spi/par/opaque master struct (3 times), and not in registered_master struct. I added shutdown function where the data is.