spi: spidev: Add an entry for lwn,bk4-spi

spi: spidev: Add an entry for lwn,bk4-spi

Currently, the compatible string used for Liebherr's BK4 external SPI
controller 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']
...

Add a more specific "lwn,bk4-spi" compatible string entry for this
device to fix the problem.

The original "lwn,bk4" is kept to keep compatibility with old DTBs.

Signed-off-by: Fabio Estevam <festevam@denx.de>
Link: https://patch.msgid.link/20241023120015.1049008-2-festevam@gmail.com
Signed-off-by: Mark Brown <broonie@kernel.org>