powerpc64: Don't guard ISA 3.0 partition table setup with hw_direct_map
PowerISA 3.0 eliminated the 64-bit bridge mode which allowed 32-bit kernels
to run on 64-bit AIM/Book-S hardware. Since therefore only a 64-bit kernel
can run on this hardware, and 64-bit native always has the direct map, there
is no need to guard it.