dt-bindings: firmware: scm: Refactor compatibles and clocks

When the binding was written all "future" platforms required three
clocks, so the default compatible (qcom,scm) was defined to require
this. But as history shows all "future" platforms actually lack required
clocks. Given how the binding is written these compatibles have to be
added as an exception to the default.

Refactor the description of compatible to define that a platform
compatible should be given, followed by the fallback of qcom,scm. Also
refactor the description of the clocks in a way that this does not need
to be updated as new platform specific compatibles are added.

Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Reviewed-by: Stephen Boyd <sboyd@kernel.org>
Signed-off-by: Andy Gross <andy.gross@linaro.org>