dt-bindings: can: fsl,flexcan: add uint32 reference to clock-frequency property

This patch adds the missing reference to the clock-frequency property. The
driver uses a of_property_read_u32() to read the property to mark it as uint32.

Fixes: e5ab9aa7e4 ("dt-bindings: can: flexcan: convert fsl,*flexcan bindings to yaml")
Link: https://lore.kernel.org/r/20201119073357.2858925-1-mkl@pengutronix.de
Cc: Joakim Zhang <qiangqing.zhang@nxp.com>
Cc: Oleksij Rempel <ore@pengutronix.de>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>

Documentation/devicetree/bindings/net/can/fsl,flexcan.yaml

@@ -57,6 +57,7 @@ properties:
       - const: per
 
   clock-frequency:
+    $ref: /schemas/types.yaml#/definitions/uint32
     description: |
       The oscillator frequency driving the flexcan device, filled in by the
       boot loader. This property should only be used the used operating system