IIO ADC support for AD7923

This patch adds support for Analog Devices AD7923 ADC in the IIO Subsystem.

Signed-off-by: Patrick Vasseur <patrick.vasseur@c-s.fr>
Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
Reviewed-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>