clk: meson: meson8b: add the cts_amclk clocks

Add the I2S master clock also referred as cts_amclk. The setup for this
clock is identical to GXBB, so this ports commit 4087bd4b

 ("clk:
meson: gxbb: add cts_amclk") to the Meson8/Meson8b/Meson8m2 clock
driver.

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