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.

Also, lower the clock for the ADC as it allows to have more stable reads and
this is the speed used by atmel_tsadcc.
It lowers the maximum throughput rate from 440000 samples per second to 12958
samples per second. It shouldn't be an issue as the CPU is not able to keep up
reading samples at that frequency.
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>
Acked-by: Jonathan Cameron <jic23@kernel.org>
Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>