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/MPC8610HPCD_defconfig b/configs/MPC8610HPCD_defconfig
index ca8cb95..b4e053a 100644
--- a/configs/MPC8610HPCD_defconfig
+++ b/configs/MPC8610HPCD_defconfig
@@ -6,6 +6,7 @@
 CONFIG_OF_STDOUT_VIA_ALIAS=y
 CONFIG_BOOTDELAY=10
 # CONFIG_CONSOLE_MUX is not set
+CONFIG_SYS_STDIO_DEREGISTER=y
 CONFIG_HUSH_PARSER=y
 CONFIG_CMD_I2C=y
 CONFIG_CMD_USB=y