mfd: wm8994: Allow to configure Speaker Mode Pullup from dts

Configuring Speaker Mode Pullup was already supported in pdata, but not
in the dts.

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>