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>

board/wandboard/README

@@ -3,8 +3,8 @@ U-Boot for Wandboard
 
 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/