[PATCH] powerpc: Call find_legacy_serial_ports() if we enable CONFIG_SERIAL_8250

In setup_arch and setup_system call find_legacy_serial_ports() if we
build in support for 8250 serial ports instead of basing it on PPC_MULTIPLATFORM.

Signed-off-by: Kumar Gala <galak@kernel.crashing.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>