This patch adds support for programming drain latency registers of Pondicherry
memory arbiter of Valleyview.

v2: clarify function names (Daniel)
    fix summary typo (Daniel)
v3: add parens (Ben)
    make drain function return bool (Ben)
Acked-by: NBen Widawsky <ben@bwidawsk.net>
Signed-off-by: NGajanan Bhat <gajanan.bhat@intel.com>
Reviewed-by: NShobhit Kumar <shobhit.kumar@intel.com>
Reviewed-by: NVijay Purushothaman <vijay.a.purushothaman@intel.com>
Reviewed-by: NJesse Barnes <jesse.barnes@intel.com>
Signed-off-by: NJesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>