riscv: Use of_get_cpu_hwid()

Replace open coded parsing of CPU nodes' 'reg' property with
of_get_cpu_hwid().

Cc: Paul Walmsley <paul.walmsley@sifive.com>
Cc: Palmer Dabbelt <palmer@dabbelt.com>
Cc: Albert Ou <aou@eecs.berkeley.edu>
Cc: linux-riscv@lists.infradead.org
Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20211006164332.1981454-9-robh@kernel.org