powerpc/mpc512x: add platform code for MPC5125.

Tested on MPC5125 Tower evaluation board with
mpc512x_defconfig compile configuration.

In detail, supports for:
- PSC / UART
- RTC
- ETH
- DIU
- I2C

Signed-off-by: Matteo Facchinetti <matteo.facchinetti@sirius-es.it>
Signed-off-by: Anatolij Gustschin <agust@denx.de>