i2c: bitbang: Add library for software driven I2C

This library implements the I2C single master protocol in software.
It supports the Standard-mode and Fast-mode speeds and doesn't support
optional protocol feature like 10-bit addresses or clock stretching.

Change-Id: I375d572a83714522421f2967dc414b3bec169e95
Signed-off-by: Jon Medhurst <tixy@linaro.org>