x86: Move CONFIG_X86_SERIAL to Kconfig

Move this config option to Kconfig and tidy up.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
diff --git a/include/configs/minnowmax.h b/include/configs/minnowmax.h
index 655ce3d..d0a8aa6 100644
--- a/include/configs/minnowmax.h
+++ b/include/configs/minnowmax.h
@@ -16,7 +16,7 @@
 #define CONFIG_SYS_MONITOR_LEN		(1 << 20)
 #define CONFIG_ARCH_EARLY_INIT_R
 
-#define CONFIG_X86_SERIAL
+#define CONFIG_SMSC_LPC47M
 
 #define CONFIG_PCI_MEM_BUS		0xd0000000
 #define CONFIG_PCI_MEM_PHYS		CONFIG_PCI_MEM_BUS