mmc: sdhci-brcmstb: add hs400_downgrade callback for bcm2712

The attached PHY performs parameter validation, so the switch from HS200
to HS (before selecting HS400/HS400es) with a 200MHz clock fails to
update pad timings and results in CRC errors from the card.

Underclocking the interface is safe, so do that in the downgrade callback.

Signed-off-by: Jonathan Bell <jonathan@raspberrypi.com>