cavs: fix clock frequencies for cAVS v1.8, v2.0 and v2.5

SOF uses different SSP clock rates on different cAVS versions. Frequency
tables are provided in platform_ssp_freq[] arrays in clk.c for each
version and the default entry is selected by the SSP_DEFAULT_IDX index.

Signed-off-by: Guennadi Liakhovetski <guennadi.liakhovetski@linux.intel.com>