mfd: wm8994: Add a bunch of missing defaults/readables

Ever since this commit:

commit d4807ad2
regmap: Check readable regs in _regmap_read

Regmap will refuse to read a register which is not marked as readable,
this has highlighted a number of controls in this driver which are not
marked as readable/missing defaults.

This patch corrects the situation, by adding the missing
readables/defaults.

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