MAINTAINERS: add entry for AXI SPI Engine

The AXI SPI Engine driver has been in the kernel for many years but has
lacked a proper maintainers entry. This adds a new entry for the driver
and the devicetree bindings.

Signed-off-by: David Lechner <dlechner@baylibre.com>
Link: https://lore.kernel.org/r/20231117-axi-spi-engine-series-1-v1-2-cc59db999b87@baylibre.com


Signed-off-by: Mark Brown <broonie@kernel.org>