iio:adc:ad7923: Add support for the ad7904/ad7914/ad7924

The ad7924 is software compatible with the ad7923. The ad7904 and ad7914 are the
8 and 10 bit version of the ad7924.

While we are at it also drop the "with temperature sensor" from the Kconfig
entry, since the chips do not have a temperature sensor.

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