dts/bindings: use const to validate #<FOO>-cells

In most cases #<FOO>-cells should be a constant.  For example in spi
controller #address-cells should be 1, and #size-cells should be 0.

Use the const attribute to specify such single known values.  Add const
value to missing bindings which have cells.

Signed-off-by: Kumar Gala <kumar.gala@linaro.org>

dts/bindings/riscv/openisa,rv32m1-pcc.yaml

@@ -22,5 +22,8 @@ properties:
     label:
       category: required
 
+    "#clock-cells":
+      const: 1
+
 "#cells":
   - name