spi: axi-spi-engine improvements

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 first series mostly doing some housekeeping:
* Convert device tree bindings to yaml.
* Add a MAINTAINERS entry.
* Clean up probe and remove using devm.
* Separate message state from driver state.
* Add support for cs_off and variable word size.

Once this series is applied, we will follow up with a second series of
general improvements, and then finally a 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