iio: adc: at91: Add support for Touchscreen Switches Closure Time

iio: adc: at91: Add support for Touchscreen Switches Closure Time

On newer components compatible with the at91sam9x5, the Touchscreen
Switches Closure Time or TSSCTIM value of the Touchscreen Mode Register is
not filled at all.
On some hardware, having no time indicated for it may lead to incoherent
values and jitter.
We fix this time to 10us as it is usually difficult to retrieve impedance
values from LCD manufacturers.

Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>