powerpc/8xx: rewrite flush_instruction_cache() in C

On PPC8xx, flushing instruction cache is performed by writing
in register SPRN_IC_CST. This registers suffers CPU6 ERRATA.
The patch rewrites the fonction in C so that CPU6 ERRATA will
be handled transparently

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