iio: adc: tlc4541: add support for TI tlc4541 adc

This adds TI's tlc4541 16-bit ADC driver. Which is a single channel
ADC. Supports raw and trigger buffer access.
Also supports the tlc3541 14-bit device, which has not been tested.
Implementation of the tlc3541 is fairly straight forward thou.

Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Phil Reid <preid@electromag.com.au>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>