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.

Signed-off-by: Vladimir Ermakov <vooon341@gmail.com>
Signed-off-by: Matteo Facchinetti <matteo.facchinetti@sirius-es.it>
Signed-off-by: Anatolij Gustschin <agust@denx.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>