dt-bindings: misc: lwn,bk4-spi: Add binding

dt-bindings: misc: lwn,bk4-spi: Add binding

Add a lwn,bk4-spi.yaml binding for Liebherr's BK4 external SPI controller.

Currently, the compatible string used for this device is "lwn,bk4",
which is the same as the board compatible string documented at fsl.yaml.

This causes several dt-schema warnings:

make dtbs_check DT_SCHEMA_FILES=fsl.yaml
...

['lwn,bk4'] is too short
'lwn,bk4' is not one of ['tq,imx8dxp-tqma8xdp-mba8xx']
'lwn,bk4' is not one of ['tq,imx8qxp-tqma8xqp-mba8xx']
'lwn,bk4' is not one of ['armadeus,imx1-apf9328', 'fsl,imx1ads']
...

Use a more specific "lwn,bk4-spi" compatible string for this
device.

Signed-off-by: Fabio Estevam <festevam@denx.de>
Link: https://patch.msgid.link/20241023120015.1049008-1-festevam@gmail.com
Signed-off-by: Mark Brown <broonie@kernel.org>