platform: sifive: fu540: Use standard value string for cpu node status

Per device tree spec, the standard value string for the "status"
property of a cpu node is either "okay" or "disabled". "masked"
was once used but it is unfortunately a spec violation.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Acked-by: Anup Patel <anup.patel@wdc.com>