Add support for CONFIG_SERIAL_MULTI on MPC5xxx

Patch by Martin Krause, 8 Jun 2006

This patch supports two serial consoles on boards with
a MPC5xxx CPU. The console can be switched at runtime
by setting stdin, stdout and stderr to the desired serial
interface (serial0 or serial1). The PSCs to be used as
console port are definded by CONFIG_PSC_CONSOLE
and CONFIG_PSC_CONSOLE2.
See README.serial_multi for details.