Fix 64-bit build of libofw.

Adjust a couple of printf() lines that deal with dumping out addresses
to cast to uintmax_t.

This allows building a 64-bit libofw for use in things like a future
Petitboot loader for PowerPC64, and other FDT platforms that require
a 64-bit loader binary and want to use forth.

Sponsored by: Tag1 Consulting, Inc.