powerpc: fix a few pmap related functions to return correct types

While experimenting with changing boolean_t to another type, I noticed
that several powerpc pmap related functions returned the wrong type:
boolean_t instead of int.

Fix several declarations and definitions to match the actual pmap
function types: pmap_dev_direct_mapped_t and pmap_ts_referenced_t.

MFC after: 3 days

(cherry picked from commit d142601887524fc595ed0f5e6a69419727222f0a)