rockchip: px30: bind sub-nodes for GRF (syscon)

There are some sub-nodes under the grf DT nodes, so add bind callback
function in syscon_px30 driver to scan them recursively.

Fixes: e9ccb2f5 ("rockchip: add px30 architecture core")
Signed-off-by: Quentin Schulz <quentin.schulz@cherry.de>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>