video: mcux_csi: set a dedicated init priority for video_mcux_csi

Set a dedicated priority for the video_mcux_csi instead of using the
default kernel device init priority. This allows initializing the device
in a sequence that matches the devicetree hirearchy compared to mt9m114.

Fixes the error:

ERROR: /soc/csi@402bc000 POST_KERNEL 50 <
	/soc/i2c@403f0000/mt9m114@48 POST_KERNEL 60

found using:

$ west build -p -b mimxrt1064_evk samples/subsys/video/capture \
		-DCONFIG_CHECK_INIT_PRIORITIES=y

Signed-off-by: Fabio Baltieri <fabiobaltieri@google.com>