i2c: Adds i2c_transfer_async API for I2C

i2c_transfer_async is the asynchronous version of i2c_transfer where
the completion of the transfer is notified to the caller using a callback.

This can be used in conjuction with the common callbacks and datatypes
in async.h for directly doing an async transfer with an IPC object
to notify a thread.

Signed-off-by: Tom Burdick <thomas.burdick@intel.com>