mfd: wm5110: Add mic detect level registers

The mic detect level registers are used from common code to configure
the button detection code. Add these registers to the defaults list and
mark them as readable for wm5110.

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