i2c: import SMBus stuff from linux ...

This patch imports i2c_smbus_{read,write}* functions from linux-3.15.

Here is a quote from linux.git/Documentation/i2c/smbus-protocol:

    If you write a driver for some I2C device, please try to use the SMBus
    commands if at all possible (if the device uses only that subset of the
    I2C protocol). This makes it possible to use the device driver on both
    SMBus adapters and I2C adapters (the SMBus command set is automatically
    translated to I2C on I2C adapters, but plain I2C commands can not be
    handled at all on most pure SMBus adapters).

So having i2c_smbus_{read,write}* functions in barebox makes
porting of linux i2c client drivers much easier as many drivers rely on
these smbus functions.

Signed-off-by: Antony Pavlov <antonynpavlov@gmail.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>