drivers: i2s: i2s_litex: Calculate offsets from DT

To support both 8-bit and 32-bit Control/Status register variants, register
offsets need to be calculated from device tree.

Updated register data in device tree to the 32-bit CSR variant.
Renamed defines to be similar to other LiteX drivers.

Changed frequencies in clock-outputs nodes, so i2s/litex sample works.

Signed-off-by: Michal Sieron <msieron@internships.antmicro.com>