spmi: Linux driver framework for SPMI

SPMI (System Power Management Interface) is a specification
developed by the MIPI (Mobile Industry Process Interface) Alliance
optimized for the real time control of Power Management ICs (PMIC).

SPMI is a two-wire serial interface that supports up to 4 master
devices and up to 16 logical slaves.  A physical PMIC device may
consist of multiple logical slave interfaces.

The framework supports message APIs, multiple busses (1 controller
per bus) and multiple clients/slave devices per controller.

Change-Id: Ib6319047b4ab9cea6dfb0879312c4c1f63462439
Signed-off-by: Kenneth Heitke <kheitke@codeaurora.org>