spi: dt-bindings: nxp,imx94-xspi: Document imx94 xspi

spi: dt-bindings: nxp,imx94-xspi: Document imx94 xspi

Document imx94 xspi that supports interface to serial flash
supporting following features:

- Single-bit SPI, Dual SPI, Quad SPI and Octal SPI.
- Single Data Rate or Double Data Rate modes.
- Direct memory mapping of all AHB memory accesses to the
  chip system memory space.
- Multi-master AHB accesses with priority.

Reviewed-by: Krzysztof Kozlowski <krzk@kernel.org>
Signed-off-by: Haibo Chen <haibo.chen@nxp.com>
Link: https://patch.msgid.link/20251216-xspi-v7-1-282525220979@nxp.com
Signed-off-by: Mark Brown <broonie@kernel.org>