mmc: mmci: Add bindings to operate CMD, CK, CKIN pins as GPIO

Add DT bindings to describe GPIO line associated with CMD, CK, CKIN pins.

Signed-off-by: Marek Vasut <[e-mail]>
Cc: Alexandre Torgue <[e-mail]>
Cc: Ludovic Barre <[e-mail]>
Reviewed-by: Linus Walleij <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Ulf Hansson <[e-mail]>

Documentation/devicetree/bindings/mmc/arm,pl18x.yaml

···
       driver to sample the receive data (for example with a voltage switch
       transceiver).
 
+  st,cmd-gpios:
+    maxItems: 1
+    description:
+      The GPIO matching the CMD pin.
+
+  st,ck-gpios:
+    maxItems: 1
+    description:
+      The GPIO matching the CK pin.
+
+  st,ckin-gpios:
+    maxItems: 1
+    description:
+      The GPIO matching the CKIN pin.
+
+dependencies:
+  st,cmd-gpios: [ "st,use-ckin" ]
+  st,ck-gpios: [ "st,use-ckin" ]
+  st,ckin-gpios: [ "st,use-ckin" ]
+
 unevaluatedProperties: false
 
 required: