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 <[e-mail]>
Cc: Rob Herring <[e-mail]>
Reviewed-by: Rob Herring <[e-mail]> # for fixing the dtc warnings

Documentation/devicetree/bindings/riscv/cpus.yaml

···
   - |
     // Example 2: Spike ISA Simulator with 1 Hart
     cpus {
-            cpu@0 {
-                    device_type = "cpu";
-                    reg = <0>;
-                    compatible = "riscv";
-                    riscv,isa = "rv64imafdc";
-                    mmu-type = "riscv,sv48";
-                    interrupt-controller {
-                            #interrupt-cells = <1>;
-                            interrupt-controller;
-                            compatible = "riscv,cpu-intc";
-                    };
-            };
+        #address-cells = <1>;
+        #size-cells = <0>;
+        cpu@0 {
+                device_type = "cpu";
+                reg = <0>;
+                compatible = "riscv";
+                riscv,isa = "rv64imafdc";
+                mmu-type = "riscv,sv48";
+                interrupt-controller {
+                        #interrupt-cells = <1>;
+                        interrupt-controller;
+                        compatible = "riscv,cpu-intc";
+                };
+        };
     };
 ...