Preparatory move toward the ultimate goal of moving pan-subdevice stuff
into include/linux.

Cc: ScottFang@viatech.com.cn
Cc: JosephChan@via.com.tw
Cc: Harald Welte <laforge@gnumonks.org>
Acked-by: NFlorian Tobias Schandinat <FlorianSchandinat@gmx.de>
Signed-off-by: NJonathan Corbet <corbet@lwn.net>