dt-bindings: clock: Document external clocks for MSM8998 gcc

The global clock controller on MSM8998 can consume a number of external
clocks.  Document them.

For 7180 and 8150, the hardware always exists, so no clocks are truly
optional.  Therefore, simplify the binding by removing the min/max
qualifiers to clocks.  Also, fixup an example so that dt_binding_check
passes.

Signed-off-by: Jeffrey Hugo <jhugo@codeaurora.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lkml.kernel.org/r/1576595987-10043-1-git-send-email-jhugo@codeaurora.org


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