mfd: wm8997: Add registers for high power mode

Some output configurations can require a 50Mhz SYSCLK which requires
DCVDD to be 1.8V. This patch adds the registers necessary for
supporting this operational mode.

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