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.

This is a snapshot from msm-4.19
commit 9a702c565e5a ("Use 'GPL-2.0-only' as the
SPDX-License-Identifier tag").

Change-Id: I28a4905e1666c06cb2c06d00f56ef66981621b2d
Signed-off-by: Nicholas Troast <ntroast@codeaurora.org>
Signed-off-by: Guru Das Srinagesh <gurus@codeaurora.org>
Signed-off-by: David Collins <collinsd@codeaurora.org>