mfd: wm8994: Export symbols ready for built-in => modular build

We'd like to be able to build the WM8994 family as modules, but the
build will fail if the external symbols aren't exported first.

Acked-by: Charles Keepax <ckeepax@opensource.wolfsonmicro.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>