mfd: wm5110: Add registers for headphone short circuit control

Add the registers necessary to enable/disable the headphone short
circuit protection.

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>