i2c: xiic: Add platform module alias

The missing "platform" alias is required for the mgb4 v4l2 driver to load
the i2c controller driver when probing the HW.

Signed-off-by: Martin Tůma <martin.tuma@digiteqautomotive.com>
Acked-by: Michal Simek <michal.simek@amd.com>
Signed-off-by: Wolfram Sang <wsa@kernel.org>