i2c: Move bitbang_i2c.h to include/hw/i2c/

The bitbang i2c implementation is also useful for other device models
such as DDC in display controllers. Move the header to include/hw/i2c/
to allow it to be used from other device models and adjust users of
this include. This also reverts commit 2b4c1125 which is no longer
needed.

Signed-off-by: BALATON Zoltan <balaton@eik.bme.hu>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Tested-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Acked-by: Corey Minyard <cminyard@mvista.com>
Message-id: 5d1fe4db846ab9be4b77ddb0d43cc74cd200a003.1561028123.git.balaton@eik.bme.hu
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>