drm: xlnx: zynqmp_dpsub: Fix kernel doc

Fix two kernel doc warnings introduced by the recent DP audio patch:

- Add a doc line for the new "audio" field
- Remove a reference to zynqmp_dpsub.c from zynqmp.rst, as the .c file
  no longer has structured comments

Fixes: 3ec5c157 ("drm: xlnx: zynqmp_dpsub: Add DP audio support")
Closes: https://lore.kernel.org/all/20241220154208.720d990b@canb.auug.org.au/


Reviewed-by: Vishal Sagar <vishal.sagar@amd.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20241220-xilinx-dp-audio-doc-fix-v1-1-cc488996e463@ideasonboard.com