powerpc: Introduce a new config symbol to control 16550 early debug code

The previous change by Kumar Gala in this area led to legacy_serial.c
and udbg_16550.c being built as modules when CONFIG_SERIAL_8250=m.
Fix this by introducing a new symbol, CONFIG_PPC_UDBG_16550, to
control whether these files get built, and arrange for it to be selected
for those platforms that need it.

Signed-off-by: Paul Mackerras <paulus@samba.org>

arch/powerpc/kernel/setup_64.c

@@ -472,9 +472,7 @@ void __init setup_system(void)
 	 * hash table management for us, thus ioremap works. We do that early
 	 * so that further code can be debugged
 	 */
-#ifdef CONFIG_SERIAL_8250
 	find_legacy_serial_ports();
-#endif
 
 	/*
 	 * "Finish" the device-tree, that is do the actual parsing of