[media] rcar_vin: Add DT support for r8a7793 and r8a7794 SoCs

Based on platform device work by Matsuoka-san.

Signed-off-by: Yoshihiro Kaneko <ykaneko0929@gmail.com>
Signed-off-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>