dt-bindings: pinctrl: stm32: Fix 'st, syscfg' description field

As there is only one item "st,syscfg" this commit moves phandle description
fields under "description" tag. It'll fix a validation issue seen  during
stm32 DT check.
Signed-off-by: Alexandre Torgue <alexandre.torgue@st.com>
Signed-off-by: Rob Herring <robh@kernel.org>

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

@@ -40,10 +40,9 @@ properties:
     allOf:
       - $ref: "/schemas/types.yaml#/definitions/phandle-array"
     description: Should be phandle/offset/mask
-    items:
-      - description: Phandle to the syscon node which includes IRQ mux selection.
-      - description: The offset of the IRQ mux selection register.
-      - description: The field mask of IRQ mux, needed if different of 0xf.
+      - Phandle to the syscon node which includes IRQ mux selection.
+      - The offset of the IRQ mux selection register.
+      - The field mask of IRQ mux, needed if different of 0xf.
 
   st,package:
     allOf: