mfd: wm8994: Ensure that the whole MFD is built into a single module

The MFD part of wm8994 consists of three files wm8994-core.c,
wm8994-irq.c and wm8994-regmap.c only wm8994-core.c has a
MODULE_DESCRIPTION / LICENSE. These were clearly intended to be built
as a single module, but currently are not. This will lead to a tainted
kernel when loading modules for wm8894-irq.c and wm8994-regmap.c because
are missing a license.

This patch fixes this issue by grouping the three files together into a
single module.

Reported-by: Peter Robinson <pbrobinson@gmail.com>
Signed-off-by: Charles Keepax <ckeepax@opensource.wolfsonmicro.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>