Kconfig: Add description for CMD_POWEROFF

Add poweroff description to Kconfig to make it selectable
via menuconfig.

Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/cmd/Kconfig b/cmd/Kconfig
index 270cff6..412bf24 100644
--- a/cmd/Kconfig
+++ b/cmd/Kconfig
@@ -266,7 +266,9 @@
 	  Extract a part of a multi-image.
 
 config CMD_POWEROFF
-	bool
+	bool "poweroff"
+	help
+	  Poweroff/Shutdown the system
 
 endmenu