mfd: rn5t618: Mark ADC control register volatile

BugLink: https://bugs.launchpad.net/bugs/1864775

commit 2f3dc25c upstream.

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>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Khalid Elmously <khalid.elmously@canonical.com>
Signed-off-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>

drivers/mfd/rn5t618.c

@@ -28,6 +28,7 @@ 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: