clk: meson: gxbb: add cts_i958 clock

This adds the cts_i958 clock to control the clock source of the spdif
output block. This mux is not explicitly mentionned in the documentation
but it is critical to the spdif dai. It is used to select whether the clock
source of the spdif output is cts_amclk (when data are taken from i2s
buffer) or the cts_mclk_i958 (when data are taken from the spdif buffer)

Acked-by: Michael Turquette <mturquette@baylibre.com>
Signed-off-by: Jerome Brunet <jbrunet@baylibre.com>