ARM: at91: updated the at91_dt_defconfig with support for the ADS7846

This provides touchscreen support to the at91sam9261ek

Signed-off-by: Jean-Jacques Hiblot <jjhiblot@traphandler.com>
Acked-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>
Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>