spi: Helper for deriving timeout values

Merge series from Miquel Raynal <miquel.raynal@bootlin.com>:

I recently came across an issue with the Atmel spi controller driver
which would stop my transfers after a too small timeout when performing
big transfers (reading a 4MiB flash in one transfer). My initial idea
was to derive a the maximum amount of time a transfer would take
depending on its size and use that as value to avoid erroring-out when
not relevant. Mark wanted to go further by creating a core helper doing
that, based on the heuristics from the sun6i driver.

Here is a small series of 3 patches doing exactly that.