examples: Add ad57xx dac example

[ahaslam2]

This adds an iio example app for the ad57xx dac.
The example uses the buffer interface to generate a triangular wave of aprox 1 khz.

PR Type

• Bug fix (a change that fixes an issue)
• [x ] New feature (a change that adds new functionality)
• Breaking change (a change that affects other repos or cause CIs to fail)

PR Checklist

• [x ] I have conducted a self-review of my own code changes
• I have commented new code, particulary complex or unclear areas
• I have checked that I did not intoduced new warnings or errors (CI output)
• I have checked that components that use libiio did not get broken
• I have updated the documentation accordingly (GitHub Pages, READMEs, etc)

[rgetz]

Coverity points out:

Variable 'p_inc' is assigned a value that is never used.

examples/ad57xx-iiostream.c

129	while (!stop) {
130		int32_t *p_dat, *p_end;
131		ptrdiff_t p_inc = sample_size;
132
133		block = iio_stream_get_next_block(txstream);

This should have been caught, but we aren't doing #1223

-Robin