dt-bindings: clock: stm32mp1: support disabled fixed clock

Add precision for disabled fixed clock in stm32mp1 binding.
Signed-off-by: NPatrick Delaunay <patrick.delaunay@st.com>

doc/device-tree-bindings/clock/st,stm32mp1.txt

@@ -164,8 +164,10 @@ used to define the state of associated ST32MP1 oscillators:
   - clk-csi
 
 At boot the clock tree initialization will
-  - enable oscillators present in device tree
+  - enable oscillators present in device tree and not disabled
+    (node with status="disabled"),
   - disable HSI oscillator if the node is absent (always activated by bootrom)
+    and not disabled (node with status="disabled").
 
 Optional properties :