mfd: wm8994: Allow to configure CS/ADDR Pulldown from dts

For designs where CS/ADDR pin is floating, it is useful to
allow dts to define whether to keep internal pull down or not.

Signed-off-by: Alberto Panizzo <alberto@amarulasolutions.com>
Signed-off-by: Anthony Brandon <anthony@amarulasolutions.com>
Acked-by: Charles Keepax <ckeepax@opensource.cirrus.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>