ARM: dts: vf610-bk4: Use the more specific "lwn,bk4-spi"

ARM: dts: vf610-bk4: Use the more specific "lwn,bk4-spi"

Currently, the compatible string used for the spidev 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 to fix the problem.

Signed-off-by: Fabio Estevam <festevam@denx.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>