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>
diff --git a/arch/powerpc/kernel/Makefile b/arch/powerpc/kernel/Makefile
index 6e03b59..0faf95c 100644
--- a/arch/powerpc/kernel/Makefile
+++ b/arch/powerpc/kernel/Makefile
@@ -55,7 +55,7 @@
 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)