bcm2835-sdhost and bcm2835-mmc ignore the max-frequency DT property

[pelwell]

The DT bindings for MMC interfaces include a "max-frequency" property that is intended to limit SD bus clock speeds, but neither the bcm2835-sdhost driver nor the bcm2835-mmc driver honour this property.

In the same vein, there is no reason why the sdhost driver couldn't allow the overclock mechanism to also permit underclocking.

See https://www.raspberrypi.org/forums/viewtopic.php?f=107&t=203156.