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

Cc: Yoshinori Sato <ysato@users.sourceforge.jp>
Cc: Rich Felker <dalias@libc.org>
Cc: linux-sh@vger.kernel.org
Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20211006164332.1981454-10-robh@kernel.org