dt-bindings: riscv: resolve 'make dt_binding_check' warnings

Rob pointed out that one of the examples in the RISC-V 'cpus' YAML
schema results in warnings from 'make dt_binding_check'.  Fix these.

While here, make the whitespace in the second example consistent
with the first example.
Signed-off-by: Paul Walmsley <paul.walmsley@sifive.com>
Cc: Rob Herring <robh@kernel.org>
Reviewed-by: Rob Herring <robh@kernel.org> # for fixing the dtc warnings

Documentation/devicetree/bindings/riscv/cpus.yaml

@@ -152,6 +152,8 @@ examples:
   - |
     // Example 2: Spike ISA Simulator with 1 Hart
     cpus {
+        #address-cells = <1>;
+        #size-cells = <0>;
         cpu@0 {
                 device_type = "cpu";
                 reg = <0>;