media: v4l: cadence: add VIDEO_V4L2 dependency

The cadence media drivers can be built-in while the v4l2 core is a loadable
module. This is a mistake and leads to link errors:

drivers/media/v4l2-core/v4l2-fwnode.o: In function `v4l2_async_register_subdev_sensor_common':
v4l2-fwnode.c:(.text+0x12f0): undefined reference to `v4l2_async_subdev_notifier_register'
v4l2-fwnode.c:(.text+0x1304): undefined reference to `v4l2_async_register_subdev'
v4l2-fwnode.c:(.text+0x1318): undefined reference to `v4l2_async_notifier_unregister'
v4l2-fwnode.c:(.text+0x1338): undefined reference to `v4l2_async_notifier_cleanup'
cdns-csi2rx.c:(.text+0x9f8): undefined reference to `v4l2_subdev_init'
cdns-csi2rx.c:(.text+0xa78): undefined reference to `v4l2_async_register_subdev'
drivers/media/platform/cadence/cdns-csi2tx.o: In function `csi2tx_remove':
cdns-csi2tx.c:(.text+0x88): undefined reference to `v4l2_async_unregister_subdev'
drivers/media/platform/cadence/cdns-csi2tx.o: In function `csi2tx_probe':
cdns-csi2tx.c:(.text+0x884): undefined reference to `v4l2_subdev_init'
cdns-csi2tx.c:(.text+0xa9c): undefined reference to `v4l2_async_register_subdev'

An explicit Kconfig dependency on VIDEO_V4L2 avoids the problem.

Fixes: 1fc3b37f

 ("media: v4l: cadence: Add Cadence MIPI-CSI2 RX driver")

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Maxime Ripard <maxime.ripard@bootlin.com>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>