spi: axi-spi-engine: improvements round 2

Merge series from David Lechner <dlechner@baylibre.com>:

We are working towards adding support for the offload feature [1] of the
AXI SPI Engine IP core. Before we can do that, we want to make some
general fixes and improvements to the driver. In order to avoid a giant
series with 35+ patches, we are splitting this up into a few smaller
series.

This is a continuation of the work started in [2] which has been applied
to spi/for-6.8 [3]. This series must be applied on top of that series to
apply cleanly.

Once this series is applied, we will follow up with the 3rd series that
implements the offload support. The offload support will also involve
the IIO subsystem (a new IIO driver will depend on the new SPI offload
feature), so I'm mentioning this now in case we want to do anything
ahead of time to prepare for that (e.g. putting all of these changes on
a separate branch).

[1]: https://wiki.analog.com/resources/fpga/peripherals/spi_engine/offload
[2]: https://lore.kernel.org/linux-spi/20231117-axi-spi-engine-series-1-v1-0-cc59db999b87@baylibre.com/
[3]: https://git.kernel.org/pub/scm/linux/kernel/git/broonie/spi.git/log/?h=for-6.8