mfd: wm5110: Give new AIF2 registers defaults and mark as readable

The registers associated with the new channels on AIF2 were accidentally
missing defaults and not marked as readable this patch fixes this.

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