drm/bridge: tc358767: Add precious register SYSSTAT

This is the single register which clears its value upon read operation.

Signed-off-by: Alexander Stein <alexander.stein@ew.tq-group.com>
Reviewed-by: Robert Foss <rfoss@kernel.org>
Signed-off-by: Robert Foss <rfoss@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20231212075257.75084-7-alexander.stein@ew.tq-group.com