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/configs/gwventana_defconfig b/configs/gwventana_defconfig
index c1a3c1c..67753ad 100644
--- a/configs/gwventana_defconfig
+++ b/configs/gwventana_defconfig
@@ -17,6 +17,7 @@
 CONFIG_OF_BOARD_SETUP=y
 CONFIG_SYS_EXTRA_OPTIONS="IMX_CONFIG=arch/arm/imx-common/spl_sd.cfg,MX6QDL"
 CONFIG_BOOTDELAY=3
+# CONFIG_SYS_STDIO_DEREGISTER is not set
 # CONFIG_DISPLAY_BOARDINFO is not set
 CONFIG_SPL=y
 CONFIG_SPL_STACK_R=y