powerpc, 5xx: remove some "5xx" remains

we removed 5xx support. So delete some forgotten remains.

Signed-off-by: Heiko Schocher <hs@denx.de>

drivers/serial/serial.c

@@ -223,7 +223,6 @@ void serial_initialize(void)
 	max3100_serial_initialize();
 	mcf_serial_initialize();
 	ml2_serial_initialize();
-	mpc5xx_serial_initialize();
 	mpc85xx_serial_initialize();
 	mpc8xx_serial_initialize();
 	mxc_serial_initialize();