mpc512x: Fix problem with I2C access before relocation

This is needed for the upcoming esd MECP5123 board port which uses
I2C EEPROM for environment storage.

Signed-off-by: Stefan Roese <sr@denx.de>
Cc: Reinhard Arlt <reinhard.arlt@esd-electronics.com>
Acked-by: Heiko <Schocher&lt;hs@denx.de>