samples: drivers: i2s: Add LiteX I2S sample

This commit adds the LiteX I2S devices usage example with:
 - i2s rx and tx initialization,
 - i2s rx and tx configuration,
 - i2s rx receiving,
 - i2s tx sending.

An application is a simple sound loopback - it allows to connect
a music source and a receiver such as headseat and listen to it.

Signed-off-by: Pawel Sagan <psagan@internships.antmicro.com>
Signed-off-by: Mateusz Holenko <mholenko@antmicro.com>