powerpc: Move flush_instruction_cache() prototype in asm/cacheflush.h

flush_instruction_cache() belongs to the cache flushing function
family.

Move its prototype in asm/cacheflush.h

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/993445b5227e8ca2f0e38bcc9ea3dfea6e865920.1597384512.git.christophe.leroy@csgroup.eu