dt-bindings: serial: stm32: Use 'type: object' instead of false for 'additionalProperties'

To use additional properties 'bluetooth' on serial, need replace false with
'type: object' for 'additionalProperties' to make it as a node, else will
run into dtbs_check warnings.

'arch/arm/boot/dts/stm32h750i-art-pi.dt.yaml: serial@40004800:
'bluetooth' does not match any of the regexes: 'pinctrl-[0-9]+'

Fixes: af1c2d81 ("dt-bindings: serial: Convert STM32 UART to json-schema")
Reported-by: kernel test robot <lkp@intel.com>
Tested-by: Valentin Caron <valentin.caron@foss.st.com>
Signed-off-by: dillon min <dillon.minfei@gmail.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/1616757302-7889-8-git-send-email-dillon.minfei@gmail.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Documentation/devicetree/bindings/serial/st,stm32-uart.yaml

@@ -80,7 +80,8 @@ required:
   - interrupts
   - clocks
 
-additionalProperties: false
+additionalProperties:
+  type: object
 
 examples:
   - |