clock-frequency is only restricted by the upper limit of 400 kHz.

Found with:
$ DT_SCHEMA_FILES=Documentation/devicetree/bindings/i2c/i2c-imx.yaml make dtbs_check
...
arch/arm64/boot/dts/freescale/imx8mq-librem5-r2.dt.yaml: i2c@30a20000: clock-frequency:0:0: 387000 is not one of [100000, 400000]
	From schema: linux/Documentation/devicetree/bindings/i2c/i2c-imx.yaml
...

Fixes: 4bdc4434 ("dt-bindings: i2c: Convert imx i2c to json-schema")
Signed-off-by: David Heidelberg <david@ixit.cz>
Reviewed-by: Oleksij Rempel <o.rempel@pengutronix.de>
Link: https://lore.kernel.org/r/20211029141134.66170-1-david@ixit.czSigned-off-by: Rob Herring <robh@kernel.org>