greybus: i2c-gb: split out into a stand-alone kernel module.

This splits the i2c-gb protocol into a stand-alone kernel module.

It's not going to stay in this fashion for long, this was done to test
the "can a protcol be loaded later" logic.  Future refactoring is going
to move the gpbridge protocols to a separate kernel module, where this
protocol is going to live.

But for now, split it out, it is good to test with, and shows a bug in
gbsim at the moment.

Reviewed-by: Alex Elder <elder@linaro.org>
Signed-off-by: Greg Kroah-Hartman <greg@kroah.com>