[U-Boot] rockchip: rk3399-puma: reduce sd card max-frequency to 40MHz

[Philipp Tomsich]

> Some SanDisk Ultra cards trigger intermittent errors on detection
> resulting in an -EOPNOTSUPP, when running at 50MHz.
> 
> Waveform analysis suggest that the level shifters that are used on the
> RK3399-Q7 module (for voltage translation between the on-module
> voltages and the 3.3V required on the card-edge) don't handle clock
> rates at or above 48MHz properly. This change reduces the maximum
> frequency on the external SD-interface to 40MHz (for a safety margin
> of 20%).
> 
> Reported-by: Jakob Unterwurzacher <jakob.unterwurzacher at theobroma-systems.com>
> Signed-off-by: Philipp Tomsich <philipp.tomsich at theobroma-systems.com>
> Tested-by: Christoph Muellner <christoph.muellner at theobroma-systems.com>
> ---
> 
>  arch/arm/dts/rk3399-puma.dtsi | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 

Applied to u-boot-rockchip, thanks!