ARC: Fix earlycon build breakage

Commit ffb7fcd6 ("ARC: Dynamically determine BASE_BAUD from DeviceTree")
breaks arc:defconfig build:

       drivers/built-in.o: In function `of_setup_earlycon':
       (.init.text+0xb3e): undefined reference to `arc_early_base_baud'
       drivers/built-in.o: In function `setup_earlycon':
       (.init.text+0xcd0): undefined reference to `arc_early_base_baud'
       make: *** [vmlinux] Error 1

BASE_BAUD is only required for earlycon, which should depend on
CONFIG_SERIAL_EARLYCON.
Reported-by: Guenter Roeck <linux@roeck-us.net>
Tested-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Peter Hurley <peter@hurleysoftware.com>
Signed-off-by: Vineet Gupta <vgupta@synopsys.com>

arch/arc/kernel/devtree.c

@@ -17,7 +17,7 @@
 #include <asm/clk.h>
 #include <asm/mach_desc.h>
 
-#ifdef CONFIG_SERIAL_8250_CONSOLE
+#ifdef CONFIG_SERIAL_EARLYCON
 
 static unsigned int __initdata arc_base_baud;