mfd: rn5t618: Mark ADC control register volatile

There is a bit which gets cleared after conversion.

Fixes: 9bb9e29c

 ("mfd: Add Ricoh RN5T618 PMIC core driver")
Signed-off-by: Andreas Kemnade <andreas@kemnade.info>
Signed-off-by: Lee Jones <lee.jones@linaro.org>