alpha: use asm-generic/cacheflush.h

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

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Cc: Richard Henderson <rth@twiddle.net>
Cc: Ivan Kokshaysky <ink@jurassic.park.msu.ru>
Cc: Matt Turner <mattst88@gmail.com>
Link: http://lkml.kernel.org/r/20200515143646.3857579-9-hch@lst.de


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