media: rcar-csi2: Handle per-SoC number of channels

The R-Car CSI-2 interface has a number of selectable 'channels' that
provides pixel data to the VINs during image acquisition.

Each channel can be used to match a CSI-2 data type and a CSI-2 virtual
channel to be routed to output path.

Different SoCs have different number of channels, with R-Car E3 being the
notable exception supporting only 2 of them.

Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Jacopo Mondi <jacopo+renesas@jmondi.org>
Signed-off-by: Hans Verkuil <hansverk@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>