v4l: rcar-fcp: Add an API to retrieve the FCP device

The new rcar_fcp_get_device() function retrieves the struct device
related to the FCP device. This is useful to handle DMA mapping through
the right device.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Reviewed-by: Kieran Bingham <kieran.bingham+renesas@ideasonboard.com>
Signed-off-by: Kieran Bingham <kieran.bingham+renesas@ideasonboard.com>
Acked-by: Mauro Cavalho Chehab <mchehab@s-opensource.com>