dt-bindings: crypto: fsl,sec-v4.0-mon: Add missing type for "linux,keycode"

The "linux,keycode" property is missing a type probably because it was
confused with the common property "linux,keycodes".

Reviewed-by: Conor Dooley <conor.dooley@microchip.com>
Link: https://lore.kernel.org/r/20230613201231.2826352-1-robh@kernel.org


Signed-off-by: Rob Herring <robh@kernel.org>