Convert CONFIG_CMD_GSC to Kconfig

This converts the following to Kconfig:
   CONFIG_CMD_GSC

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/board/gateworks/gw_ventana/Kconfig b/board/gateworks/gw_ventana/Kconfig
index c577984..5d1bae4 100644
--- a/board/gateworks/gw_ventana/Kconfig
+++ b/board/gateworks/gw_ventana/Kconfig
@@ -14,4 +14,12 @@
 	help
 	  Provides access to EEPROM configuration on Gateworks Ventana
 
+config CMD_GSC
+	bool "Enable the 'gsc' command"
+	help
+	  Provides access to the GSC configuration:
+
+	     gsc sleep - sleeps for a period of seconds
+	     gsc wd - enables / disables the watchdog
+
 endif