dt-bindings: marvell,mmp2: Add clock id for the I2S clocks

There are two of these on a MMP2.

Signed-off-by: Lubomir Rintel <lkundrak@v3.sk>
Acked-by: Rob Herring <robh@kernel.org>
Link: https://lkml.kernel.org/r/20200519224151.2074597-4-lkundrak@v3.sk


Signed-off-by: Stephen Boyd <sboyd@kernel.org>