mfd: rn5t618: Mark ADC control register volatile

There is a bit which gets cleared after conversion.

Fixes: 9bb9e29 ("mfd: Add Ricoh RN5T618 PMIC core driver")
Signed-off-by: Andreas Kemnade <andreas@kemnade.info>
Signed-off-by: Lee Jones <lee.jones@linaro.org>

Author: akemnade

drivers/mfd/rn5t618.c

@@ -26,6 +26,7 @@ static bool rn5t618_volatile_reg(struct device *dev, unsigned int reg)
 	case RN5T618_WATCHDOGCNT:
 	case RN5T618_DCIRQ:
 	case RN5T618_ILIMDATAH ... RN5T618_AIN0DATAL:
+	case RN5T618_ADCCNT3:
 	case RN5T618_IR_ADC1 ... RN5T618_IR_ADC3:
 	case RN5T618_IR_GPR:
 	case RN5T618_IR_GPF: