i2c: xgene: Fix missing code of DTB support

In DTB case, i2c-core doesn't create slave device which is installed
on i2c-xgene bus because of missing code in this driver.
This patch fixes this issue.

Signed-off-by: Tin Huynh <tnhuynh@apm.com>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>