bindings: Add sck-idle-input to spi-gpio

Signed-off-by: Phil Elwell <phil@raspberrypi.com>

Documentation/devicetree/bindings/spi/spi-gpio.yaml

@@ -43,6 +43,10 @@ properties:
       with no chip select is connected.
     $ref: /schemas/types.yaml#/definitions/uint32
 
+  sck-idle-input:
+    description: Make SCK an input when inactive.
+    type: boolean
+
   # Deprecated properties
   gpio-sck: false
   gpio-miso: false