Initialize earlier macppc's and powerpc64's pmap_hash_lock

Initialize earlier macppc's and powerpc64's pmap_hash_lock

At boot, the powerpc64 kernel was calling
  pmap_bootstrap -> pmap_kenter_pa -> mtx_enter(&pmap_hash_lock)
before it did
  pmap_init -> mtx_init(&pmap_hash_lock, IPL_HIGH)

Change from mtx_init to MUTEX_INITIALIZER.  This allows an option
WITNESS kernel to boot without warning of an uninitialized mutex.

Also change macppc's pmap_hash_lock from __ppc_lock_init to
PPC_LOCK_INITIALIZER, though WITNESS doesn't see this lock.

ok mpi@