Convert CONFIG_SYS_STDIO_DEREGISTER to Kconfig

This converts the following to Kconfig:
   CONFIG_SYS_STDIO_DEREGISTER

This option should never be enabled in SPL, so use
CONFIG_IS_ENABLED(SYS_STDIO_DEREGISTER) when checking the option.

Signed-off-by: Simon Glass <sjg@chromium.org>
[trini: Re-sync]
Signed-off-by: Tom Rini <trini@konsulko.com>
diff --git a/include/stdio_dev.h b/include/stdio_dev.h
index 13f76be..e4fc8b1 100644
--- a/include/stdio_dev.h
+++ b/include/stdio_dev.h
@@ -100,7 +100,7 @@
 int stdio_init(void);
 
 void	stdio_print_current_devices(void);
-#ifdef CONFIG_SYS_STDIO_DEREGISTER
+#if CONFIG_IS_ENABLED(SYS_STDIO_DEREGISTER)
 int stdio_deregister(const char *devname, int force);
 int stdio_deregister_dev(struct stdio_dev *dev, int force);
 #endif