dt-bindings: spi: stm32: add bindings regarding stm32h7 spi slave

Update the spi-stm32 binding yaml regarding to the SPI slave support.

Signed-off-by: Alain Volmat <alain.volmat@foss.st.com>
Change-Id: I3117b2f9b4c86a5e8b76e63491e1eb1a8f7bbb03
Reviewed-on: https://gerrit.st.com/c/mpu/oe/st/linux-stm32/+/257544
Reviewed-by: CITOOLS <MDG-smet-aci-reviews@list.st.com>
Reviewed-by: Valentin CARON <valentin.caron@foss.st.com>
Reviewed-by: CIBUILD <MDG-smet-aci-builds@list.st.com>
Reviewed-by: Amelie DELAUNAY <amelie.delaunay@foss.st.com>
Tested-by: Valentin CARON <valentin.caron@foss.st.com>

Documentation/devicetree/bindings/spi/st,stm32-spi.yaml

@@ -27,6 +27,7 @@ allOf:
     then:
       properties:
         st,spi-midi-ns: false
+        spi-slave: false
 
 properties:
   "#address-cells": true
@@ -62,6 +63,17 @@ properties:
       - const: rx
       - const: tx
 
+  spi-slave:
+    description:
+      The SPI controller acts as a slave, instead of a master.
+
+  cs-gpios:
+    description:
+      In case of spi-slave not defined, cs-gpios behave as defined in
+      spi-controller.yaml.
+      In case of spi-slave defined, if <0>, indicate that SS should be
+      detected via the dedicated HW pin
+
 patternProperties:
   "^[a-zA-Z][a-zA-Z0-9,+\\-._]{0,63}@[0-9a-f]+$":
     type: object