iio: adc: palmas: add dt support for autoconversion

iio: adc: palmas: add dt support for autoconversion

Add DT parsing for auto conversion configurations on palmas-adc.
The autoconversion subnode is require as
 gpadc {
	::::
	auto_conv0 {
		// Auto-conversion 0 parameters
	};
	auto_conv1 {
		// Auto-conversion 1 parameters
	};
	::::::::
};

Change-Id: Id730213f6d5c685efef65cee9c74c4e33c81c66c
Signed-off-by: Laxman Dewangan <ldewangan@nvidia.com>
Reviewed-on: http://git-master/r/392401