openrisc: use asm-generic/cacheflush.h

openrisc: use asm-generic/cacheflush.h

OpenRISC 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: Jonas Bonn <jonas@southpole.se>
Cc: Stefan Kristiansson <stefan.kristiansson@saunalahti.fi>
Cc: Stafford Horne <shorne@gmail.com>
Link: http://lkml.kernel.org/r/20200515143646.3857579-16-hch@lst.de
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>