MIPS: AR7: Fix build failures when CONFIG_SERIAL_8250 is not enabled

This patch fixes the following build failure when CONFIG_SERIAL_8250
is not enabled in the kernel configuration:
arch/mips/ar7/built-in.o: In function 'ar7_register_devices':
platform.c:(.init.text+0x61c): undefined reference to 'early_serial_setup'
platform.c:(.init.text+0x61c): relocation truncated to fit: R_MIPS_26 against 'early_serial_setup'
platform.c:(.init.text+0x68c): undefined reference to 'early_serial_setup'
platform.c:(.init.text+0x68c): relocation truncated to fit: R_MIPS_26 against 'early_serial_setup'

Signed-off-by: Florian Fainelli <florian@openwrt.org>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>

arch/mips/ar7/platform.c

@@ -480,6 +480,7 @@ static void __init detect_leds(void)
 static int __init ar7_register_devices(void)
 {
 	int res;
+#ifdef CONFIG_SERIAL_8250
 	static struct uart_port uart_port[2];
 
 	memset(uart_port, 0, sizeof(struct uart_port) * 2);
@@ -511,7 +512,7 @@ static int __init ar7_register_devices(void)
 		if (res)
 			return res;
 	}
-
+#endif /* CONFIG_SERIAL_8250 */
 	res = platform_device_register(&physmap_flash);
 	if (res)
 		return res;