mfd: qcom-i2c-pmic: add option to skip irq control

Some MFD PMIC configurations do not support interrupts. Make the interrupt
controller optional to support these PMIC configrations.

Without an interrupt controller this driver simply passes a regmap to
its children.

CRs-Fixed: 1037797
Change-Id: I52b909dd226494987c79ed106ffdf9892d966ef1
Signed-off-by: Nicholas Troast <ntroast@codeaurora.org>