drivers: i2s: Add LiteX I2S controller driver

This introduces LiteX I2S driver supporting the following features:
    - 8,16,24,32 bit sample width,
    - mono/stereo sound,
    - different sample frequencies
    - big/little-endian data format,
    - concatenated channels mode (for selected sample widths only),
    - slave/master mode operation.

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