[POWERPC] Remove flush_dcache_all export

Removes the flush_dcache_all export for non coherent platforms.
We removed the last in-kernel user of this years ago in arch/ppc
so it no longer serves a purpose. Plus, it breaks the build
at the moment.

Signed-off-by: Matt Porter <mporter@embeddedalley.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>