dt-bindings: clock: Convert fixed-clock binding to json-schema

Convert the fixed-clock binding to DT schema format using json-schema.

Cc: Michael Turquette <mturquette@baylibre.com>
Cc: Stephen Boyd <sboyd@kernel.org>
Cc: linux-clk@vger.kernel.org
Signed-off-by: Rob Herring <robh@kernel.org>
[sboyd@kernel.org: Drop full stop on title]
Signed-off-by: Stephen Boyd <sboyd@kernel.org>