i2c: i2c_sbcon: Driver for ARM's SBCon 2-wire serial interface

SBCon is a simple device which allows directly setting and getting the
hardware state of two-bit serial interfaces like I2C. Therefore to be
useable we need to drive the lines with the appropriate protocol under
software control.

Change-Id: If9000bb75f7b0ad7bbb256b1cb38cc70fa6ca8ea
Signed-off-by: Jon Medhurst <tixy@linaro.org>