powerpc: Use of_get_cpu_hwid()

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

Cc: Michael Ellerman <mpe@ellerman.id.au>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: Paul Mackerras <paulus@samba.org>
Cc: linuxppc-dev@lists.ozlabs.org
Signed-off-by: Rob Herring <robh@kernel.org>
Acked-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20211006164332.1981454-8-robh@kernel.org