iio:ad7476: Add support for ad7273/ad7274/ad7276/ad7277/ad7278

The ad7276/ad7277/ad7278 are similar to the ad7476/ad7477/ad7478 but have the
same number of leading zeros as the ad7940. The ad7273/ad7274 have a extra pin
for VREF where as for the ad7276/ad7277/ad7278 VREF is taken from VDD, but
otherwise they are compatible to the ad7276/ad7277.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>