dt-bindings: mfd: st,stmfx: Remove I2C unit name

Remove the I2C unit name to fix the following build warning with
'make dt_binding_check':

Warning (unit_address_vs_reg): /example-0/i2c@0: node has a unit name, but no reg or ranges property
Signed-off-by: Fabio Estevam <festevam@gmail.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>

Documentation/devicetree/bindings/mfd/st,stmfx.yaml

@@ -93,7 +93,7 @@ required:
 examples:
   - |
     #include <dt-bindings/interrupt-controller/arm-gic.h>
-    i2c@0 {
+    i2c {
       #address-cells = <1>;
       #size-cells = <0>;
       stmfx@42 {