[media] soc_camera: rcar_vin: add device tree support for r8a7792

Simply document new compatibility string.
As a previous patch adds a generic R-Car Gen2 compatibility string
there appears to be no need for a driver updates.

By documenting this compat string it may be used in DTSs shipped, for
example as part of ROMs. It must be used in conjunction with the Gen2
fallback compat string. At this time there are no known differences between
the r8a7792 IP block and that implemented by the driver for the Gen2
fallback compat string. Thus there is no need to update the driver as the
use of the Gen2 fallback compat string will activate the correct code in
the current driver while leaving the option for r8a7792-specific driver
code to be activated in an updated driver should the need arise.

Signed-off-by: Simon Horman <horms+renesas@verge.net.au>
Acked-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>