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/at91rm9200ek_ram_defconfig b/configs/at91rm9200ek_ram_defconfig
index e042f0b..7c00ede 100644
--- a/configs/at91rm9200ek_ram_defconfig
+++ b/configs/at91rm9200ek_ram_defconfig
@@ -3,6 +3,7 @@
 CONFIG_TARGET_AT91RM9200EK=y
 CONFIG_SYS_EXTRA_OPTIONS="RAMBOOT"
 CONFIG_BOOTDELAY=3
+# CONFIG_SYS_STDIO_DEREGISTER is not set
 # CONFIG_DISPLAY_CPUINFO is not set
 # CONFIG_DISPLAY_BOARDINFO is not set
 CONFIG_HUSH_PARSER=y