iio:ad5064: Add support for ltc2633 and similar devices

The Linear Technology LTC2631, LTC2633 and LTC2635 are very similar
to the AD5064 device, in particular the LTC2627.

This patch adds support for those devices. Only the LTC2633 has been
tested, which is the 2-channel variant. The LTC2631 is the 1-channel,
and the LTC2635 the 4-channel version. The actual DAC resolution depends
on the exact chip type and can be 12, 10 or 8 bits, using the upper bits
so this has no effect on the register map. The internal reference is set
to 2.5V on "L" versions, and it's 4.096V for "H" versions.

Datasheets:
    LTC2631: http://www.linear.com/docs/26553
    LTC2633: http://www.linear.com/docs/39529
    LTC2635: http://www.linear.com/docs/28754



Signed-off-by: Mike Looijmans <mike.looijmans@topic.nl>
Reviewed-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>