MIPS: Fix build error due to multiple prom_putchar() definitions.

This can happen if both the generic 8250 and another early console
driver are enable.  Fixed by using an auxilliary kconfig symbol to
restrict that choice.

Signed-off-by: Ralf Baechle <ralf@linux-mips.org>
diff --git a/arch/mips/Kconfig.debug b/arch/mips/Kconfig.debug
index b147e70..25de292 100644
--- a/arch/mips/Kconfig.debug
+++ b/arch/mips/Kconfig.debug
@@ -21,13 +21,17 @@
 	  unless you want to debug such a crash.
 
 config EARLY_PRINTK_8250
-	bool "8250/16550 and compatible serial early printk driver"
-	depends on EARLY_PRINTK
-	default n
+	bool
+	depends on EARLY_PRINTK && USE_GENERIC_EARLY_PRINTK_8250
+	default y
 	help
+	  "8250/16550 and compatible serial early printk driver"
 	  If you say Y here, it will be possible to use a 8250/16550 serial
 	  port as the boot console.
 
+config USE_GENERIC_EARLY_PRINTK_8250
+	bool
+
 config CMDLINE_BOOL
 	bool "Built-in kernel command line"
 	default n