dt-bindings: mmc: add no-mmc-hs400 flag

HS400 requires a data strobe line in addition to the usual MMC signal
lines. If a board design neglects to wire up this signal, HS400 mode is
not available, even if both the controller and the eMMC are claiming to
support this mode. Add a DT flag to allow boards to disable the HS400
support in this case.
Signed-off-by: Lucas Stach <dev@lynxeye.de>
Acked-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20210510190400.105162-1-l.stach@pengutronix.deSigned-off-by: Ulf Hansson <ulf.hansson@linaro.org>

Documentation/devicetree/bindings/mmc/mmc-controller.yaml

@@ -220,6 +220,11 @@ properties:
     description:
       eMMC HS400 enhanced strobe mode is supported
 
+  no-mmc-hs400:
+    $ref: /schemas/types.yaml#/definitions/flag
+    description:
+      All eMMC HS400 modes are not supported.
+
   dsr:
     description:
       Value the card Driver Stage Register (DSR) should be programmed