powerpc: use asm-generic/cacheflush.h

Power needs almost no cache flushing routines of its own.  Rely on
asm-generic/cacheflush.h for the defaults.

Also remove the pointless __KERNEL__ ifdef while we're at it.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Michael Ellerman <mpe@ellerman.id.au>
Link: http://lkml.kernel.org/r/20200515143646.3857579-17-hch@lst.de


Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>