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/whistler_defconfig b/configs/whistler_defconfig
index 4874d5b..2c83661 100644
--- a/configs/whistler_defconfig
+++ b/configs/whistler_defconfig
@@ -5,6 +5,7 @@
 CONFIG_DEFAULT_DEVICE_TREE="tegra20-whistler"
 CONFIG_OF_SYSTEM_SETUP=y
 CONFIG_CONSOLE_MUX=y
+CONFIG_SYS_STDIO_DEREGISTER=y
 CONFIG_HUSH_PARSER=y
 CONFIG_SYS_PROMPT="Tegra20 (Whistler) # "
 CONFIG_CMD_BOOTZ=y