rockchip: rk3399-puma: reduce sd card max-frequency to 40MHz

rockchip: rk3399-puma: reduce sd card max-frequency to 40MHz

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@theobroma-systems.com>
Signed-off-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
Tested-by: Christoph Muellner <christoph.muellner@theobroma-systems.com>