dt-bindings: clk: fixed-mmio-clock: Convert to YAML

Convert the binding documentatoin for fixed-mmio-clock to YAML.

Signed-off-by: Marek Behún <kabel@kernel.org>
Link: https://lore.kernel.org/r/20210903152615.31453-1-kabel@kernel.org


Reviewed-by: Rob Herring <robh@kernel.org>
Reviewed-by: Stephen Boyd <sboyd@kernel.org>
Signed-off-by: Stephen Boyd <sboyd@kernel.org>