dts: bindings: entropy: stm32: Add description to clock property

This addition should help users to better understand potential issues
with domain clock configuration.

Signed-off-by: Erwan Gouriou <erwan.gouriou@linaro.org>

dts/bindings/rng/st,stm32-rng.yaml

@@ -13,6 +13,13 @@ properties:
 
   clocks:
     required: true
+    description: |
+         Specifies the clock line to be enabled in clock controller as well as
+         the clock domain used, for instance:
+                   <&rcc STM32_SRC_MSI CLK48_SEL(3)> /* RNG clock domain set to MSI */
+         A correctly configured domain clock is required to allow the integrated low
+         sampling clock detection mecanism to behave properly.
+         In provided example, MSI should be configured to provide 48Mhz clock.
 
   nist-config:
     type: int