dt-bindings: i2c: imx: add SoC specific compatible strings for layerscape

Add chip specific compatible strings for layerscape platforms to make it
possible to do SoC specific fix-ups in the driver.

Signed-off-by: Li Yang <leoyang.li@nxp.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Wolfram Sang <wsa@kernel.org>