mfd: arizona: Mark additional registers as volatile

Mark some additional registers as volatile. The write sequencer control
registers should not be cached, as we don't ever want their value
synchronised as this might cause a write sequence to be accidentally
initiated.

Additionally, the DAC_COMP registers require special preconditions to
write so there values wouldn't be updated accurately during a register
sync.

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