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/mx6cuboxi_defconfig b/configs/mx6cuboxi_defconfig
index 3f00773..22173cc 100644
--- a/configs/mx6cuboxi_defconfig
+++ b/configs/mx6cuboxi_defconfig
@@ -12,6 +12,7 @@
 CONFIG_SPL_WATCHDOG_SUPPORT=y
 CONFIG_VIDEO=y
 CONFIG_SYS_EXTRA_OPTIONS="IMX_CONFIG=arch/arm/imx-common/spl_sd.cfg,MX6QDL"
+# CONFIG_SYS_STDIO_DEREGISTER is not set
 CONFIG_SPL=y
 CONFIG_HUSH_PARSER=y
 CONFIG_CMD_BOOTZ=y