Docs/ABI/testing: add DAMON sysfs interface ABI document

Docs/ABI/testing: add DAMON sysfs interface ABI document

This commit adds DAMON sysfs interface ABI document under
Documentation/ABI/testing.

Link: https://lkml.kernel.org/r/20220228081314.5770-14-sj@kernel.org
Signed-off-by: SeongJae Park <sj@kernel.org>
Cc: David Rientjes <rientjes@google.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Shuah Khan <skhan@linuxfoundation.org>
Cc: Xin Hao <xhao@linux.alibaba.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>