[U-Boot] rockchip: clk: rk3399: fix SPI clock code for SPI3

[Philipp Tomsich]

> On the RK3399, SPI3 is special (it's configured via PMUGRF). The
> implementation of the clock-setup for SPI3 was never supported (and
> the comments reflected this), but the implementation did not correctly
> handle this.
> 
> This fixes the code to match the comments above it and returns an
> error when trying to configure SPI3.
> 
> Signed-off-by: Philipp Tomsich <philipp.tomsich at theobroma-systems.com>
> ---
> 
>  drivers/clk/rockchip/clk_rk3399.c | 18 ++++++++++++++----
>  1 file changed, 14 insertions(+), 4 deletions(-)
> 

Acked-by: Philipp Tomsich <philipp.tomsich at theobroma-systems.com>