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/Makefile

@@ -55,7 +55,7 @@ obj-$(CONFIG_BOOTX_TEXT)	+= btext.o
 obj-$(CONFIG_6xx)		+= idle_6xx.o
 obj-$(CONFIG_SMP)		+= smp.o
 obj-$(CONFIG_KPROBES)		+= kprobes.o
-obj-$(CONFIG_SERIAL_8250)	+= legacy_serial.o udbg_16550.o
+obj-$(CONFIG_PPC_UDBG_16550)	+= legacy_serial.o udbg_16550.o
 module-$(CONFIG_PPC64)		+= module_64.o
 obj-$(CONFIG_MODULES)		+= $(module-y)
 

arch/powerpc/kernel/setup_32.c

@@ -299,9 +299,7 @@ void __init setup_arch(char **cmdline_p)
 	if (ppc_md.init_early)
 		ppc_md.init_early();
 
-#ifdef CONFIG_SERIAL_8250
 	find_legacy_serial_ports();
-#endif
 	finish_device_tree();
 
 	smp_setup_cpu_maps();

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