powerpc: doc/dts-bindings: update doc of FSL I2C bindings

This patch adds the MPC5121 to the list of supported devices,
enhances the doc of the "clock-frequency" property and removes
the obsolete "cell-index", "device_type" and "fsl-i2c" property.
Furthermore an example for the MPC5121 has been added.

Signed-off-by: Wolfgang Grandegger <wg@denx.de>
Reviewed-by: Wolfram Sang <w.sang@pengutronix.de>
Signed-off-by: Ben Dooks <ben-linux@fluff.org>