dt-bindings: clock: Create YAML schema for ICST clocks

The ICST clocks used in the ARM Integrator, Versatile and
RealView platforms are updated to use YAML schema, and two
new ICST clocks used by the Integrator IM-PD1 logical module
are added in the process.

Cc: devicetree@vger.kernel.org
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Link: https://lkml.kernel.org/r/20200219103326.81120-1-linus.walleij@linaro.org
[sboyd@kernel.org: Fix some typos]
Signed-off-by: Stephen Boyd <sboyd@kernel.org>