powerpc32: Remove clear_pages() and define clear_page() inline

clear_pages() is never used expect by clear_page, and PPC32 is the
only architecture (still) having this function. Neither PPC64 nor
any other architecture has it.

This patch removes clear_pages() and moves clear_page() function
inline (same as PPC64) as it only is a few isns

Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
Signed-off-by: Scott Wood <oss@buserror.net>