rockchip: rk3399: Move rockchip_get_cru() out of the driver

This function is called from outside the driver. It should be placed into
common SoC code. Move it.

Also rename the driver symbol to be more consistent with the other rockchip
clock drivers.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>