i2c: Document standard fault codes

Create Documentation/i2c/fault-codes to help standardize
fault/error code usage in the I2C stack.  It turns out that
returning -1 (-EPERM) for everything was not at all helpful.

Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Jean Delvare <khali@linux-fr.org>