serial/mpc52xx_uart: prepare for adding MPC5125 PSC UART support
MPC5125 PSC controller has different register layout than MPC5121.
To support MPC5125 PSC in this driver we have to provide further
psc_ops functions for SoC specific register accesses.
Add new register access functions to the psc_ops structure and
provide MPC52xx and MPC512x specific implementation for them.
Then replace remaining direct register accesses in the driver by
appropriate psc_ops function calls. The subsequent patch can now
add MPC5125 specific set of psc_ops functions.