wandboard: README: Include the quad version

Wandboard quad was not ported into U-boot at the time of writing the README.

Add it to the list of Wandboard variants.

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
diff --git a/board/wandboard/README b/board/wandboard/README
index 498db2f..1f678e1 100644
--- a/board/wandboard/README
+++ b/board/wandboard/README
@@ -3,8 +3,8 @@
 
 This file contains information for the port of U-Boot to the Wandboard.
 
-Wandboard is a development board that has two variants: one version based
-on mx6 dual lite and another one based on mx6 solo.
+Wandboard is a development board that has three variants based on the following
+SoCs: mx6 quad, mx6 dual lite and mx6 solo.
 
 For more details about Wandboard, please refer to:
 http://www.wandboard.org/