powerpc: Don't print cpu_spec->cpu_name if it's NULL

Currently we assume that if the cpu_spec has a pvr_mask then it must also have a
cpu_name. But that will change in a subsequent commit when we do CPU feature
discovery via the device tree, so check explicitly if cpu_name is NULL.

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>