powerpc/mpc5121: enable support for more PSC UARTs

MPC5121 has 12 PSC devices. Enable UART support for all of
them by defining the number of max. PSCs depending on
selection of PPC_MPC512x platform support.

Signed-off-by: Anatolij Gustschin <agust@denx.de>
Acked-by: Greg Kroah-Hartman <gregkh@suse.de>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>