mfd: wm5110: Mark register containing FLL gains as readable

The FLL gains are present on all devices and controlled from common code
shared between them. This patch adds these registers into defaults list
for wm5110 and marks them as readable.

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