drm: rcar-du: Document the rcar_du_crtc structure

struct rcar_du_group;

/**

 * struct rcar_du_crtc - the CRTC, representing a DU superposition processor

 * @crtc: base DRM CRTC

 * @clock: the CRTC functional clock

 * @extclock: external pixel dot clock (optional)

 * @mmio_offset: offset of the CRTC registers in the DU MMIO block

 * @index: CRTC software and hardware index

 * @started: whether the CRTC has been started and is running

 * @event: event to post when the pending page flip completes

 * @flip_wait: wait queue used to signal page flip completion

 * @outputs: bitmask of the outputs (enum rcar_du_output) driven by this CRTC

 * @enabled: whether the CRTC is enabled, used to control system resume

 * @group: CRTC group this CRTC belongs to

 */

struct rcar_du_crtc {

	struct drm_crtc crtc;