[POWERPC] Use const qualifiers for prom parsing utilites

The of_bus callbacks map and get_flags can be constified, as they don't
alter the range or addr arguments. of_dump_addr and of_read_addr can
also be constified.

Built for 32- and 64-bit powerpc

Signed-off-by: Jeremy Kerr <jk@ozlabs.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>