i2c: xgene: Set ACPI_COMPANION_I2C

With ACPI, i2c-core requires ACPI companion to be set in order for it
to create slave device.
This patch sets the ACPI companion accordingly.

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