dt-bindings: net: bluetooth: nxp: Add missing type for "fw-init-baudrate"

"fw-init-baudrate" is missing a type, add it. While we're here, define the
default value with a schema rather than freeform text.
Reviewed-by: Conor Dooley <conor.dooley@microchip.com>
Link: https://lore.kernel.org/r/20230613200929.2822137-1-robh@kernel.orgSigned-off-by: Rob Herring <robh@kernel.org>

Documentation/devicetree/bindings/net/bluetooth/nxp,88w8987-bt.yaml

@@ -24,11 +24,12 @@ properties:
       - nxp,88w8997-bt
 
   fw-init-baudrate:
+    $ref: /schemas/types.yaml#/definitions/uint32
+    default: 115200
     description:
       Chip baudrate after FW is downloaded and initialized.
       This property depends on the module vendor's
-      configuration. If this property is not specified,
-      115200 is set as default.
+      configuration.
 
 required:
   - compatible