global_data: Reduce size of early-malloc vars

The early malloc region is normally quite small and is certainly less
than 4GB, so use a 32-bit value for the limit and pointer. Update the
comments for clarity while we are here.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/include/asm-generic/global_data.h b/include/asm-generic/global_data.h
index 253dd21..b09d4bc 100644
--- a/include/asm-generic/global_data.h
+++ b/include/asm-generic/global_data.h
@@ -299,13 +299,13 @@
 	 */
 	unsigned long malloc_base;
 	/**
-	 * @malloc_limit: limit address of early malloc()
+	 * @malloc_limit: maximum size of early malloc()
 	 */
-	unsigned long malloc_limit;
+	unsigned int malloc_limit;
 	/**
-	 * @malloc_ptr: current address of early malloc()
+	 * @malloc_ptr: currently used bytes of early malloc()
 	 */
-	unsigned long malloc_ptr;
+	unsigned int malloc_ptr;
 #endif
 #ifdef CONFIG_CONSOLE_RECORD
 	/**