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>

include/asm-powerpc/serial.h

@@ -15,6 +15,10 @@
 /* Default baud base if not found in device-tree */
 #define BASE_BAUD ( 1843200 / 16 )
 
+#ifdef CONFIG_PPC_UDBG_16550
 extern void find_legacy_serial_ports(void);
+#else
+#define find_legacy_serial_ports()	do { } while (0)
+#endif
 
 #endif /* _PPC64_SERIAL_H */