[ARM] 4353/1: AT91: Support ADS7846 touchsceen on SAM9261-EK board

Add support for the ADS7846 Touchscreen found on the Atmel
AT91SAM9261-EK board.

Original patch by Morten Larsen.

Signed-off-by: Andrew Victor <andrew@sanpeople.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>