Revert "arm64: kill flush_cache_all()"

This reverts the upstream commit <68234df4>
("arm64: kill flush_cache_all()"). This is required internally
for certain use-cases like flushing cache before reboot to
ensure all the data is available in the ramdump.

Change-Id: I6dfd240603c05053b7d341faa613dd4866bb7f62
Signed-off-by: Rohit Vaswani <rvaswani@codeaurora.org>
Signed-off-by: Elliot Berman <eberman@codeaurora.org>