mfd: wm5110: Add registers for custom write sequence triggers

This register will be needed as part of some additional support for the
headphone path on wm5110, so this patch adds the register and sets up
its regmap config.

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