drm: xlnx: zynqmp_dpsub: Add DP audio support

Add basic DisplayPort audio support.

Support non-live audio playback from two PCMs (DMA channels), and the
volume control in the audio mixer.

As older dtb files may not have the audio DMA channels defined, the
driver will just mark the audio support as disabled if the audio DMA is
missing, and will continue with only display support.

Note: Reset doesn't seem to work (ZYNQMP_DISP_AUD_SOFT_RESET). If we do
a reset, audio playback won't start again even if, afaics, we do set up
all the necessary registers. So, at the moment, resetting the audio
block in dp_dai_hw_free() is commented out.

Tested-by: Anatoliy Klymenko <anatoliy.klymenko@amd.com>
Reviewed-by: Vishal Sagar <vishal.sagar@amd.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20241023-xilinx-dp-audio-v4-3-5128881457be@ideasonboard.com