dt-bindings: Correct the FU540-C000's timebase-frequency

The documentation says that our timebase-frequency looks like this

/ {
        cpus {
                timebase-frequency = X;
        }
}

but it actually looks like this

/ {
        cpus {
                cpu@0 {
                        timebase-frequency = X;
                }
        }
}

CC: Wesley Terpstra <wesley@sifive.com>
Signed-off-by: Palmer Dabbelt <palmer@sifive.com>