drivers/fsi: Add empty fsi bus definitions

This change adds the initial (empty) fsi bus definition, and introduces
drivers/fsi/.

Signed-off-by: Jeremy Kerr <jk@ozlabs.org>
Signed-off-by: Chris Bostic <cbostic@us.ibm.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>