powerpc/mpc5121: move PSC FIFO memory init to platform code

Since PSC could also be used in other modes than UART mode
we move PSC FIFO memory initialization from serial driver to
common platform code. The initialized FIFO memory slices may
not overlap, so the most easy way would be to configure them
all at once at init time for all PSC devices. This is now done
by this patch.

Signed-off-by: Anatolij Gustschin <agust@denx.de>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>