mfd: wm8994: Allow modular build

The interfaces that used to be built in only (interrupts IIRC) no longer
are so allow the driver to be built as a module fixing arm64 allmodconfig
builds.

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