spi-imx: correct interpretation of num-cs DT property?

[Matthias Schiffer <matthias.schiffer@xxxxxxxxxxxxxxx>]

Hi,

I've recently had a discussion about the correct way for an SPI driver
to handle the num-cs property: https://lkml.org/lkml/2020/8/25/184

Since 8cdcd8aeee281 ("spi: imx/fsl-lpspi: Convert to GPIO
descriptors"), the logic of the spi-imx driver is somewhat confusing:
If fewer than 3 cs-gpios are defined, and no explicit num-cs property
exists, the driver will set num_chipselect to 3 by default, instead of
the number of cs-gpios entries.

To avoid having to specify num-cs when the number cs-gpios would
suffice, I suggested to modify the logic to the following:

- If num-cs is set, use that
- If num-cs is unset, use the number of cs-gpios
- If num-cs is unset and no cs-gpios are defined, use a driver-provided 
default (which is 3 for spi-imx; this matches the number of native CS
pins in older implementations of this SPI controller; i.MX6 and newer
support up to 4)

Also, would it make sense to add num-cs to all DTS files for boards
that actually use fewer than 3 CS pins?

At the moment, the num-cs property is not explicitly documented for the
spi-imx driver, although the driver understands it. I also suggested to
add this to the docs, which Fabio didn't deem a good idea (I don't
quite understand the reasoning here - isn't num-cs generally a useful
property to have?)

Kind regards,
Matthias