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>