Partially revert "powerpc: Remove duplicate cacheable_memcpy/memzero functions"

This partially reverts
commit 'powerpc: Remove duplicate cacheable_memcpy/memzero functions
("b05ae4ee")'

Functions cacheable_memcpy/memzero are more efficient than
memcpy/memset as they use the dcbz instruction which avoids refill
of the cacheline with the data that we will overwrite.

Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
Signed-off-by: Scott Wood <scottwood@freescale.com>