media: dt-bindings: media: renesas,drif: Fix fck definition

dt_binding_check reports the below error with the latest schema:

Documentation/devicetree/bindings/media/renesas,drif.yaml:
  properties:clock-names:maxItems: False schema does not allow 1
Documentation/devicetree/bindings/media/renesas,drif.yaml:
  ignoring, error in schema: properties: clock-names: maxItems

This patch fixes the problem.

Signed-off-by: Fabrizio Castro <fabrizio.castro.jz@renesas.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20210408202436.3706-1-fabrizio.castro.jz@renesas.com