drm/bridge: parade-ps8640: Add support for AUX channel

Implement the first version of AUX support, which will be useful as
we expand the driver to support varied use cases.

Signed-off-by: Philip Chen <philipchen@chromium.org>
Reviewed-by: Sam Ravnborg <sam@ravnborg.org>
Reviewed-by: Douglas Anderson <dianders@chromium.org>
Reviewed-by: Stephen Boyd <swboyd@chromium.org>
[dianders: whitespace fixes reported by dim apply-branch]
Signed-off-by: Douglas Anderson <dianders@chromium.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20210921110556.v6.2.I1d6ea362dc76efa77cca2b46253d31b7651eaf17@changeid