mfd: introduce I2C PMIC controller

The I2C PMIC Controller is used by multi-function PMIC devices which
communicate over the I2C bus. The controller enumerates all child nodes
as platform devices, and instantiates a regmap interface for them to
communicate over the I2C bus.

The controller also controls interrupts for all of the children platform
devices. The controller handles the summary interrupt by deciphering which
peripheral triggered the interrupt, and which of the peripheral interrupts
were triggered. Finally, it calls the interrupt handlers for each of the
virtual interrupts that were registered.

CRs-Fixed: 975120
Change-Id: I5ebe6d0bcb7c097124ba9b35c56579815dda234f
Signed-off-by: Nicholas Troast <ntroast@codeaurora.org>