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/platforms/maple/setup.c

@@ -71,9 +71,6 @@
 #define DBG(fmt...)
 #endif
 
-extern void generic_find_legacy_serial_ports(u64 *physport,
-		unsigned int *default_speed);
-
 static void maple_restart(char *cmd)
 {
 	unsigned int maple_nvram_base;