media: imx: imx7_mipi_csis: Store CSI-2 data type in format structure

Replace the register value stored in the csis_pix_format structure with
the CSI-2 data type. The register value is simply computed from the data
type using a shift. This prepares for i.MX8MP support that needs the
same data type in a different hardware register.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Acked-by: Rui Miguel Silva <rmfrfs@gmail.com>
Tested-by: Frieder Schrempf <frieder.schrempf@kontron.de>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>