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/Kconfig b/arch/powerpc/Kconfig
index 28004f0..17c1b6a 100644
--- a/arch/powerpc/Kconfig
+++ b/arch/powerpc/Kconfig
@@ -275,6 +275,7 @@
 	select PPC_I8259
 	select PPC_RTAS
 	select RTAS_ERROR_LOGGING
+	select PPC_UDBG_16550
 	default y
 
 config PPC_CHRP
@@ -284,6 +285,7 @@
 	select PPC_INDIRECT_PCI
 	select PPC_RTAS
 	select PPC_MPC106
+	select PPC_UDBG_16550
 	default y
 
 config PPC_PMAC
@@ -306,6 +308,7 @@
 	depends on PPC_MULTIPLATFORM && PPC32 && BROKEN
 	select PPC_I8259
 	select PPC_INDIRECT_PCI
+	select PPC_UDBG_16550
 	default y
 
 config PPC_MAPLE
@@ -314,6 +317,7 @@
 	select U3_DART
 	select MPIC_BROKEN_U3
 	select GENERIC_TBSYNC
+	select PPC_UDBG_16550
 	default n
 	help
           This option enables support for the Maple 970FX Evaluation Board.
@@ -324,6 +328,7 @@
 	depends on PPC_MULTIPLATFORM && PPC64
 	select PPC_RTAS
 	select MMIO_NVRAM
+	select PPC_UDBG_16550
 
 config PPC_OF
 	def_bool y
@@ -370,6 +375,10 @@
 	depends on PPC_MAPLE
 	default y
 
+config PPC_UDBG_16550
+	bool
+	default n
+
 config CELL_IIC
 	depends on PPC_CELL
 	bool