iio:ad5064: Add AD5625/AD5627/AD5645/AD5647/AD4665/AD5657 support

iio:ad5064: Add AD5625/AD5627/AD5645/AD5647/AD4665/AD5657 support

The AD5625/AD5645/AD5665 are a family of 4 channel DACs with 12-bit, 14-bit
and 16-bit precision respectively. The devices come in 3 flavors in terms
of built-in reference, either no built-in reference, built-in 1.25V
reference or built-in 2.5V reference.

The AD5627/AD5647/AD5667 are similar to the AD5625/AD5645/AD5665 except
that they have 2 instead of 4 channels.

While these new devices are mostly register map compatible with the
existing devices support by the driver some offsets and register addresses
have been shuffled around. To accommodate this introduce a new register map
layout. For the lack of a better name we will just call it version 2.

Datasheets:
	http://www.analog.com/media/en/technical-documentation/data-sheets/AD5625R_5645R_5665R_5625_5665.pdf
	http://www.analog.com/media/en/technical-documentation/data-sheets/AD5627R_5647R_5667R_5627_5667.pdf

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