csky: Use of_get_cpu_hwid()

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

Cc: Guo Ren <guoren@kernel.org>
Cc: linux-csky@vger.kernel.org
Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20211006164332.1981454-6-robh@kernel.org