[PowerPC] Use PVO_PADDR to get the PA from PVO

Use PVO_PADDR macro to get the physical address from a PVO, instead of
explicitly ANDing pvo_pte.pa with LPTE_RPGN where it is needed. Besides
improving readability, this is needed to support superpages (D25237), where
the steps to get the PA from a PVO are different.

Reviewed by: markj
Sponsored by: Eldorado Research Institute (eldorado.org.br)
Differential Revision: https://reviews.freebsd.org/D25654