ARM: at91: sam9rlek add touchscreen support through at91_adc

at91_adc now supports reading a touchscreen for ADCs without a TSMR register.
Enable touchscreen support through at91_adc. This allows to use both a
touchscreen and the remaining ADC channel at the same time.

Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>
Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>