yaffs: add interface to cleancache

Cleancache requires a small amount of code to be added
to a filesystem's implementation so that clean page
cache pages from a filesystem of that type may be
recognized and stored in/retrieved from cleancache.

Change-Id: I94c3fc8817ab66e2c54f7b2c6c474dd2321d9806
Signed-off-by: Larry Bassel <lbassel@codeaurora.org>