clk: meson: meson8b: add the cts_i958 clock

Add the cts_i958 clock to control the clock source of the spdif output
block. 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). The setup for this clock is
identical to GXBB, so this ports commit 7eaa44f6 ("clk: meson:
gxbb: add cts_i958 clock") to the Meson8/Meson8b/Meson8m2 clock driver.

Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Signed-off-by: Jerome Brunet <jbrunet@baylibre.com>