ARM: at91/dt: at91-cosino_mega2560 remove useless tsadcc node

The tsadcc node is useless as it doesn't refer to anything and the touchscreen
is handled by the adc0 node.

Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>
Acked-by: Rodolfo Giometti <giometti@linux.it>
Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>