i2c: imx: use exact SoC revision to document binding

use exact SoC revision instead of wildcard describing
the compatible property requirement in the binding
document. This will make the binding more clearer.
Signed-off-by: NJingchang Lu <b35083@freescale.com>
Acked-by: NShawn Guo <shawn.guo@linaro.org>
Signed-off-by: NWolfram Sang <wsa@the-dreams.de>

Documentation/devicetree/bindings/i2c/i2c-imx.txt

 * Freescale Inter IC (I2C) and High Speed Inter IC (HS-I2C) for i.MX
 
 Required properties:
-- compatible : Should be "fsl,<chip>-i2c"
+- compatible :
+  - "fsl,imx1-i2c" for I2C compatible with the one integrated on i.MX1 SoC
+  - "fsl,imx21-i2c" for I2C compatible with the one integrated on i.MX21 SoC
+  - "fsl,vf610-i2c" for I2C compatible with the one integrated on Vybrid vf610 SoC
 - reg : Should contain I2C/HS-I2C registers location and length
 - interrupts : Should contain I2C/HS-I2C interrupt