spi: atmel-quadspi: Refactor to allow supporting

Merge series from "Csókás, Bence" <[email protected]>:

This is based on Tudor Ambarus' submission from 2021, and my refactor
of said sumbission from roughly a month ago.
Link: https://lore.kernel.org/linux-spi/[email protected]/
Link: https://lore.kernel.org/linux-spi/[email protected]/

This time, I submit both my refactor and the diff between it and the
original submission, rebased on top of Linus' master, namely:
commit 7d40507 ("Merge tag 'vfs-6.13-rc1.fixes' of git://git.kernel.org/pub/scm/linux/kernel/git/vfs/vfs")

Tested on a SAMA5G27C, we'll soon test on a SAMA7G54 we got from
Microchip for this purpose.

Author: broonie