rockchip: ringneck-px30: enable IO domain

Enable the IO domain on Ringneck. Based on the current HW design, this
should do nothing else than making sure vccio6 iodomain is controlled by
the GRF and not GPIO0B6.

Signed-off-by: Quentin Schulz <quentin.schulz@cherry.de>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>