powerpc: powermac: Use of_get_cpu_hwid() to read CPU node 'reg'

Replace open coded reading of CPU nodes' "reg" properties with
of_get_cpu_hwid() dedicated for this purpose.

Signed-off-by: Rob Herring <robh@kernel.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20230319145931.65499-1-robh@kernel.org