i2c: add SLIMpro I2C device driver on APM X-Gene platform

Add SLIMpro I2C device driver on APM X-Gene platform. This I2C
device driver use the SLIMpro Mailbox driver to tunnel message to
the SLIMpro coprocessor to do the work of accessing I2C components.

Signed-off-by: Feng Kan <fkan@apm.com>
Signed-off-by: Hieu Le <hnle@apm.com>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>