staging: media: imx: Merge VIDEO_IMX_CSI into VIDEO_IMX_MEDIA

The VIDEO_IMX_MEDIA Kconfig symbol used to select helpers shared between
independent drivers for different i.MX SoCs, and VIDEO_IMX_MEDIA then
selects drivers specific to the i.MX5 and i.MX6. Now that i.MX7 and
i.MX8 support has moved to drivers/media/ and doesn't depend on
VIDEO_IMX_CSI, there's no need to have separate Kconfig options. Merge
VIDEO_IMX_CSI into VIDEO_IMX_MEDIA.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>