i2c-mpc: add support for the MPC512x processors from Freescale

As I2C interrupts must  be enabled for the MPC512x by the setup function
as well, "fsl,preserve-clocking" is handled in a slighly different way.
Also, the old settings are now reported calling dev_dbg(). For the
MPC512x the clock setup function of the MPC52xx can be re-used.
Furthermore, the Kconfig help has been updated and corrected.

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>