drm/bridge: tc358767: explicitly set readable registers

This map was created from register map from datasheet (section 5.1.2).
Unused registers are stated by address, so they show up in debugfs as well.

Signed-off-by: Alexander Stein <alexander.stein@ew.tq-group.com>
Reviewed-by: Neil Armstrong <neil.armstrong@linaro.org>
Signed-off-by: Neil Armstrong <neil.armstrong@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20230516071949.375264-1-alexander.stein@ew.tq-group.com