Properly set the parent device of i2c buses before registering them so
that they will show at the right place in the device tree (rather than
in /sys/devices directly.)

Signed-off-by: Jean Delvare <jdelvare@suse.de>
Cc: Dave Airlie <airlied@gmail.com>
Reviewed-by: Alex Deucher <alexdeucher@gmail.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>