powerpc/kexec: Use of_property_read_reg()

Replace open-coded parsing of "reg" property with
of_property_read_reg().

Signed-off-by: Rob Herring (Arm) <robh@kernel.org>
[mpe: Rename end to size, add comment to the bail out case]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20240702210344.722364-1-robh@kernel.org