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