dt-bindings: media: renesas,vin: Make resets optional on R-Car Gen1

The "resets" property is not present on R-Car Gen1 SoCs.
Supporting it would require migrating from renesas,cpg-clocks to
renesas,cpg-mssr.

Fixes: 905fc6b1

 ("dt-bindings: rcar-vin: Convert bindings to json-schema")
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>
Link: https://lore.kernel.org/r/217c8197efaee7d803b22d433abb0ea8e33b84c6.1619700314.git.geert+renesas@glider.be
Signed-off-by: Rob Herring <robh@kernel.org>