mmc: core: add support for disabling HS400 mode via DT

On some boards the data strobe line isn't wired up, rendering HS400
support broken, even if both the controller and the eMMC claim to
support it. Allow to disable HS400 mode via DT.

Signed-off-by: Lucas Stach <dev@lynxeye.de>
Link: https://lore.kernel.org/r/20210510190400.105162-3-l.stach@pengutronix.de


Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>