macintosh: Use of_property_read_reg() to parse "reg"

Use the recently added of_property_read_reg() helper to get the
untranslated "reg" address value.

Signed-off-by: Rob Herring <robh@kernel.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20230609182926.1763589-1-robh@kernel.org