spmi: Add MSM PMIC Arbiter SPMI controller

Qualcomm's PMIC Arbiter SPMI controller functions as a bus master and
is used to communication with one or more PMIC (slave) devices on the
SPMI bus.  The PMIC Arbiter is actually a hardware wrapper around the
SPMI controller that provides concurrent and autonomous PMIC access
to various entities that need to communicate with the PMIC.

The SPMI controller hardware handles all of the SPMI bus activity (bus
arbitration, sequence start condition, transmission of frames, etc).
This software driver uses the PMIC Arbiter register interface to
initiate command sequences on the SPMI bus.  The status register is
read to determine when the command sequence has completed and whether
or not it completed successfully.

Request Capable Slave (RCS) devices can initiate a master write
command sequence on the SPMI bus that the can be decoded by the
bus master and used to generate interrupts.

Change-Id: I037fefc946ccb5b8e0b04da856a3a96effe1c7e4
Signed-off-by: Kenneth Heitke <kheitke@codeaurora.org>
Signed-off-by: David Brown <davidb@codeaurora.org>