i2c: Add software_i2c driver for I2C debugging and emulation

This patch adds I2C emulation in software through raw toggling of the
SDA/SCL lines. Platforms need to provide bindings to toggle their
respective I2C busses for this to work (e.g. by pinmuxing them as GPIOs,
currently only enabled for Tegra).

This is mostly useful as a debugging feature, to drive unusual states on
a bus and closely monitor the device output without the need of a bus
analyzer. It provides a few functions to "wedge" an I2C bus by aborting
a transaction at certain points, which can be used to test if a system
can correctly recover from an ill-timed reboot. However, it can also
dynamically replace the existing I2C transfer functions and drive
some/all I2C transfers on the system, which might be useful if a driver
for the actual I2C controller hardware is not (yet) available.

Based on original code by Doug Anderson <dianders@chromium.org> and
Hung-ying Tyan <tyanh@chromium.org> for the ChromeOS embedded
controller project.

TEST=Spread tegra_software_i2c_init()/tegra_software_i2c_disable()
through the code and see that everything still works.

Original-Change-Id: I9ee7ccbd1efb38206669a35d0c3318af16f8be63
Original-Signed-off-by: Julius Werner <jwerner@chromium.org>
Original-Reviewed-on: https://chromium-review.googlesource.com/198791
Original-Reviewed-by: Doug Anderson <dianders@chromium.org>
Original-Reviewed-by: Tom Warren <twarren@nvidia.com>
Original-Reviewed-by: Stefan Reinauer <reinauer@chromium.org>
(cherry picked from commit 8f71503dbbd74c5298e90e2163b67d4efe3e89db)
Signed-off-by: Marc Jones <marc.jones@se-eng.com>

Change-Id: Id6c5f75bb5baaabd62b6b1fc26c2c71d9f1ce682
Reviewed-on: http://review.coreboot.org/7947
Tested-by: build bot (Jenkins)
Reviewed-by: David Hendricks <dhendrix@chromium.org>
11 files changed