arm: wandboard: move CONFIG_MXC_UART to defconfig

For using a debug UART on the Wandboard CONFIG_MXC_UART=y must be set in
the .config file.

To avoid duplicate definitions move the setting from
include/configs/wandboard.h to configs/wandboard_defconfig.

Document the debug UART settings in the README.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Reviewed-by: Fabio Estevam <festevam@gmail.com>
diff --git a/board/wandboard/README b/board/wandboard/README
index f84f205..2d514a7 100644
--- a/board/wandboard/README
+++ b/board/wandboard/README
@@ -37,3 +37,13 @@
 - Connect the serial cable to the host PC
 
 - Power up the board and U-Boot messages will appear in the serial console.
+
+Debug UART
+----------
+
+The following settings provide a debug UART for the Wandboard:
+
+CONFIG_DEBUG_UART=y
+CONFIG_DEBUG_UART_MXC=y
+CONFIG_DEBUG_UART_BASE=0x02020000
+CONFIG_DEBUG_UART_CLOCK=80000000