iio: adc: mt7622: add support for suspend/resume.

This patch supports auxadc suspend/resume flow.
Disable auxadc clk and power in suspend function.
Enable axuadc clk and power in resume function.

Signed-off-by: Zhiyong Tao <zhiyong.tao@mediatek.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>