dt-bindings: display: renesas,du: Make resets optional on R-Car H1

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

Reflect this in the DT bindings by removing the global "required:
resets".  All SoCs that do have "resets" properties already have
SoC-specific rules making it required.

Fixes: 99d66127

 ("dt-bindings: display: renesas,du: Convert binding to YAML")
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Link: https://lore.kernel.org/r/98575791b154d80347d5b78132c1d53f5315ee62.1626257936.git.geert+renesas@glider.be


Signed-off-by: Rob Herring <robh@kernel.org>