dt-bindings: clock: Add a missing include to MMP Audio Clock binding

The include file for input clock in the example was missing, breaking the
validation.

Signed-off-by: Lubomir Rintel <lkundrak@v3.sk>
Reported-by: Rob Herring <robh+dt@kernel.org>
Link: https://lore.kernel.org/r/20200605065258.567858-1-lkundrak@v3.sk
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Stephen Boyd <sboyd@kernel.org>