openrisc: Use of_get_cpu_hwid()

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

Cc: Jonas Bonn <jonas@southpole.se>
Cc: Stefan Kristiansson <stefan.kristiansson@saunalahti.fi>
Cc: Stafford Horne <shorne@gmail.com>
Cc: openrisc@lists.librecores.org
Signed-off-by: Rob Herring <robh@kernel.org>
Acked-by: Stafford Horne <shorne@gmail.com>
Link: https://lore.kernel.org/r/20211006164332.1981454-7-robh@kernel.org