dt-bindings: clock: document the fsl-flexspi-clk device

Signed-off-by: Michael Walle <michael@walle.cc>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20201108185113.31377-7-michael@walle.cc


[sboyd@kernel.org: DT bindings aren't drivers]
Signed-off-by: Stephen Boyd <sboyd@kernel.org>