powerpc: mpc512x: Remove open coded "ranges" parsing

"ranges" is a standard property, and we have common helper functions
for parsing it, so let's use the for_each_of_range() iterator.

Signed-off-by: Rob Herring <robh@kernel.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20230609183232.1767050-1-robh@kernel.org