iio: adc: at91: add suspend and resume callback

Add suspend/resume callback, support the pinctrl sleep state when
the system suspend as well.

Signed-off-by: Wenyou Yang <wenyou.yang@atmel.com>
Acked-by: Ludovic Desroches <ludovic.desroches@atmel.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>