kconfig: Add a prompt to the MAX_MEM_RANGES setting.

Without a prompt, kconfig will silently ignore attempts to set the variable
to a non-default value.

Change-Id: I095b39852540f5728889bbefd72c6ae5edfbf542
diff --git a/src/base/Kconfig b/src/base/Kconfig
index c6b1042..d91d372 100644
--- a/src/base/Kconfig
+++ b/src/base/Kconfig
@@ -25,5 +25,5 @@
         "Set to 'y' for devices without display screens"
 
 config MAX_MEM_RANGES
-	int
+	int "Max number of memory ranges"
 	default 32