mpc512x: add multi serial PSC support

Extend mpc512x serial driver to support multiple PSC ports.

Subsequent patches for PDM360NG board support make use of this
functionality by defining CONFIG_SERIAL_MULTI in the board config
file. Additionally the used PSC devices are specified by defining
e.g. CONFIG_SYS_PSC1, CONFIG_SYS_PSC4 and CONFIG_SYS_PSC6.

Support for PSC devices other than 1, 3, 4 and 6 is not added
by this patch because these aren't used currently. In the future
it can be easily added using DECLARE_PSC_SERIAL_FUNCTIONS(N) and
INIT_PSC_SERIAL_STRUCTURE(N) macros in cpu/mpc512x/serial.c.
Additionally you have to add code for registering added
devices in serial_initialize() in common/serial.c.

Signed-off-by: Anatolij Gustschin <agust@denx.de>