powerpc/mmu: Convert PowerPC pmap drivers to ifunc from kobj

With IFUNC support in the kernel, we can finally get rid of our poor-man's
ifunc for pmap, utilizing kobj. Since moea64 uses a second tier kobj as
well, for its own private methods, this adds a second pmap install function
(pmap_mmu_init()) to perform pmap 'post-install pre-bootstrap'
initialization, before the IFUNCs get initialized.

Reviewed by: bdragon