arm: mach-rockchip: bind sub-nodes for rk3399_syscon

There are some sub-nodes under the grf DT, so add bind callback
function in rk3399 syscon driver to scan them recursively.

Signed-off-by: Frank Wang <frank.wang@rock-chips.com>
Reviewed-by: Jagan Teki <jagan@amarulasolutions.com>
Tested-by: Jagan Teki <jagan@amarulasolutions.com> # roc-rk3399-pc
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>