Documentation: clock: fixed-clock: Remove unsupported 'gpios' property

Remove the 'gpios' property from the documentation as this is something that the
current fixed clock driver does not handle.

Signed-off-by: Fabio Estevam <[e-mail]>
Signed-off-by: Mike Turquette <[e-mail]>

Documentation/devicetree/bindings/clock/fixed-clock.txt

···
 Optional properties:
 - clock-accuracy : accuracy of clock in ppb (parts per billion).
 		   Should be a single cell.
-- gpios : From common gpio binding; gpio connection to clock enable pin.
 - clock-output-names : From common clock binding.
 
 Example: