i2c: Move typedef of bitbang_i2c_interface to i2c.h

i2c: Move typedef of bitbang_i2c_interface to i2c.h

Clang 3.4 considers duplicate typedef in ppc4xx_i2c.h and
bitbang_i2c.h an error even if they are identical. Move it to a common
place to allow building with this clang version.

Reported-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: BALATON Zoltan <balaton@eik.bme.hu>
Acked-by: David Gibson <david@gibson.dropbear.id.au>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>
(cherry picked from commit 2b4c1125ac3db2734222ff43c25388a16aca4a99)
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>