iio: adc: at91: remove unused include from include/mach

That include file is now only used by the at91_adc driver, remove it from
include/mach for better driver separation.

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>