riscv: treat cpu devicetree nodes without status as enabled

Follow the Linux convention and treat devicetree nodes without a status
property as enabled rather than disabled, while also allowing "ok" as a
shorthand for "okay".

Signed-off-by: Johan Hovold <johan@kernel.org>
Reviewed-by: Paul Walmsley <paul.walmsley@sifive.com>
Signed-off-by: Palmer Dabbelt <palmer@sifive.com>