media: imx: Rename imx7-mipi-csis.c to imx-mipi-csis.c

Rename the imx7-mipi-csis.c driver to remove the reference to i.MX7.

The driver is for an IP core found on i.MX7 and i.MX8 SoC, so do not
specify a SoC version number in the driver name.

Remove the references to the i.MX7 SoC in the driver symbols and expand
the driver's header with more information about the IP core the driver
controls.

Also rename the associated bindings documentation.

Signed-off-by: Jacopo Mondi <jacopo@jmondi.org>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Acked-by: Rui Miguel Silva <rmfrfs@gmail.com>
Acked-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>